Apologies, I sent the wrong file, this is the relevant log portion after compiling with the change: 

pages           > flushing realpage 133, userpage 133, subpage 133
<msubsup>
    <mi>𝑡</mi>
    <mn>0.975</mn>
    <mo>∗</mo>
   </msubsup>
<msubsup>
    <mi>𝑡</mi>
    <mn>0.025</mn>
    <mo>∗</mo>
   </msubsup>
pages           > flushing realpage 134, userpage 134, subpage 134
pages           > flushing realpage 135, userpage 135, subpage 135
structures      > bookmark: Prueba de homogeneidad
structure       > sectioning > section @ level 3 : 0.7.2 -> Prueba de homogeneidad
structures      > bookmark: Hipótesis
structure       > sectioning > subsubsection @ level 5 : 0.7.2.0.1 -> Hipótesis
structures      > bookmark: Estadístico de prueba
structure       > sectioning > subsubsection @ level 5 : 0.7.2.0.2 -> Estadístico de prueba
structures      > bookmark: Aplicación de la homogeneidad
structure       > sectioning > subsection @ level 4 : 0.7.2.1 -> Aplicación de la homogeneidad
pages           > flushing realpage 136, userpage 136, subpage 136
floatblocks     > order disturbed
floatblocks     > '51' placed (force)
pages           > flushing realpage 137, userpage 137, subpage 137
floatblocks     > order disturbed
floatblocks     > '52' placed (force)
pages           > flushing realpage 138, userpage 138, subpage 138
close source    > level 2, order 29, name 'include/content/Aplicación-prueba-ji-cuadrado.tex'
pages           > flushing realpage 139, userpage 139, subpage 139
<msubsup>
       <mo>∑</mo>
      </msubsup>
tex error       > tex error on line 85 in file ./IntroEstadísticaNP.tex:

lua error:

        registered function call [250]: ...mtx/tex/texmf-context/tex/context/base/mkxl/lxml-mms.lmt:750: attempt to index a nil value (local 'a2')
stack traceback:
        ...mtx/tex/texmf-context/tex/context/base/mkxl/lxml-mms.lmt:750: in upvalue 'expandone'
        ...mtx/tex/texmf-context/tex/context/base/mkxl/lxml-mms.lmt:284: in upvalue 'expandfenced'
        ...mtx/tex/texmf-context/tex/context/base/mkxl/lxml-mms.lmt:585: in upvalue 'expandone'
        ...mtx/tex/texmf-context/tex/context/base/mkxl/lxml-mms.lmt:798: in upvalue 'expandone'
        ...mtx/tex/texmf-context/tex/context/base/mkxl/lxml-mms.lmt:350: in upvalue 'expandone'
        ...mtx/tex/texmf-context/tex/context/base/mkxl/lxml-mms.lmt:952: in upvalue 'expand'
        ...mtx/tex/texmf-context/tex/context/base/mkxl/lxml-mms.lmt:808: in upvalue 'expandone'
        ...mtx/tex/texmf-context/tex/context/base/mkxl/lxml-mms.lmt:605: in upvalue 'expandone'
        ...mtx/tex/texmf-context/tex/context/base/mkxl/lxml-mms.lmt:952: in upvalue 'expand'
        ...mtx/tex/texmf-context/tex/context/base/mkxl/lxml-mms.lmt:899: in upvalue 'expandone'
        ...mtx/tex/texmf-context/tex/context/base/mkxl/lxml-mms.lmt:1019: in field 'verbose'
        ...mtx/tex/texmf-context/tex/context/base/mkxl/math-tag.lmt:985: in field 'gettextblob'
        ...mtx/tex/texmf-context/tex/context/base/mkxl/lpdf-tag.lmt:391: in upvalue 'makeelement'
        ...mtx/tex/texmf-context/tex/context/base/mkxl/lpdf-tag.lmt:715: in upvalue 'structures_tags_handler'
        [string "local tonut  = nodes.tonut..."]:21: in function <[string "local tonut  = nodes.tonut..."]:12>
        (...tail calls...)
        ...mtx/tex/texmf-context/tex/context/base/mkxl/node-shp.lmt:38: in function <...mtx/tex/texmf-context/tex/context/base/mkxl/node-shp.lmt:37>
        (...tail calls...)

El dom, 25 feb 2024 a las 16:41, Andres Conrado Montoya (<andresconrado@gmail.com>) escribió:
IntroEstadisticaNP-error.log: 

return {
 ["filename"]="./IntroEstadísticaNP.tex",
 ["lastcontext"]="",
 ["lastluaerror"]="registered function call [250]: ...mtx/tex/texmf-context/tex/context/base/mkxl/lxml-mms.lmt:490: attempt to index a nil value (local 'a2')\nstack traceback:\n ...mtx/tex/texmf-context/tex/context/base/mkxl/lxml-mms.lmt:490: in upvalue 'expandone'\n ...mtx/tex/texmf-context/tex/context/base/mkxl/lxml-mms.lmt:261: in upvalue 'expandfenced'\n ...mtx/tex/texmf-context/tex/context/base/mkxl/lxml-mms.lmt:373: in upvalue 'expandone'\n ...mtx/tex/texmf-context/tex/context/base/mkxl/lxml-mms.lmt:532: in upvalue 'expandone'\n ...mtx/tex/texmf-context/tex/context/base/mkxl/lxml-mms.lmt:383: in upvalue 'expandone'\n ...mtx/tex/texmf-context/tex/context/base/mkxl/lxml-mms.lmt:669: in upvalue 'expand'\n ...mtx/tex/texmf-context/tex/context/base/mkxl/lxml-mms.lmt:544: in upvalue 'expandone'\n ...mtx/tex/texmf-context/tex/context/base/mkxl/lxml-mms.lmt:383: in upvalue 'expandone'\n ...mtx/tex/texmf-context/tex/context/base/mkxl/lxml-mms.lmt:669: in upvalue 'expand'\n ...mtx/tex/texmf-context/tex/context/base/mkxl/lxml-mms.lmt:621: in upvalue 'expandone'\n ...mtx/tex/texmf-context/tex/context/base/mkxl/lxml-mms.lmt:669: in upvalue 'expand'\n ...mtx/tex/texmf-context/tex/context/base/mkxl/lxml-mms.lmt:727: in field 'verbose'\n ...mtx/tex/texmf-context/tex/context/base/mkxl/math-tag.lmt:939: in field 'gettextblob'\n ...mtx/tex/texmf-context/tex/context/base/mkxl/lpdf-tag.lmt:391: in upvalue 'makeelement'\n ...mtx/tex/texmf-context/tex/context/base/mkxl/lpdf-tag.lmt:715: in upvalue 'structures_tags_handler'\n [string \"local tonut  = nodes.tonut...\"]:21: in function <[string \"local tonut  = nodes.tonut...\"]:12>\n (...tail calls...)\n ...mtx/tex/texmf-context/tex/context/base/mkxl/node-shp.lmt:38: in function <...mtx/tex/texmf-context/tex/context/base/mkxl/node-shp.lmt:37>\n (...tail calls...)",
 ["lasttexerror"]="",
 ["linenumber"]=85,
 ["offset"]=10,
 ["skiplinenumber"]=85,
}

Also, attached you will find the log files. 


El dom, 25 feb 2024 a las 15:58, Andres Conrado Montoya (<andresconrado@gmail.com>) escribió:
Another remark: It seems this behavior is consistent: if I comment out the last equation before the one I told you before, again the next formula works fine, 

El dom, 25 feb 2024 a las 15:38, Andres Conrado Montoya (<andresconrado@gmail.com>) escribió:
A remark: A few lines above the error triggering point, I found a single digit presented as math: \math{5}. Removing this and leaving the 5 in text mode removes the error below it. 
Could it be a bug in the distribution?

El dom, 25 feb 2024 a las 15:24, Andres Conrado Montoya (<andresconrado@gmail.com>) escribió:
I keep bumping into this error, I have no idea what may be causing it, in a statistics book with lots of math: 

registered function call [250]: ...mtx/tex/texmf-context/tex/context/base/mkxl/lxml-mms.lmt:749: attempt to index a nil value (local 'a2')

I have checked everything. I have tried commenting out many things and the weird part is that it fails in a simple math expression: \math{E_{i}j}. Even a simple $y$ or \math{a+b} will trigger the error. However, this same file Im working on has many formulas before that, those work fine. I changed all the $...$ to \math{...} just in case, but with no success. 

Already tried remaking, regenerating, updating the distribution.

Do you have any ideas to work this out? I don't know how to provide a MWE for this because it's a very big project (statistics book).  

I have tried 2 versions, the system-wide one, just upgraded: 
mtx-context     | ConTeXt Process Management 1.06
mtx-context     | current version: 2024.02.22 18:31

And a less recent version, attached to the project:
mtx-context     | ConTeXt Process Management 1.05
mtx-context     | current version: 2024.02.14 13:38


Any help will be much appreciated. 

--
----------------------------------------
Los fines no justifican los medios, porque la medida verdadera de nuestro carácter está dada por los medios que estamos dispuestos a utilizar, no por los fines que proclamamos.
----------------------------------------

“You develop an instant global consciousness, a people orientation, an intense dissatisfaction with the state of the world, and a compulsion to do something about it. From out there on the moon, international politics look so petty. You want to grab a politician by the scruff of the neck and drag him a quarter of a million miles out and say, ‘Look at that, you son of a bitch.’” — Apollo 14 astronaut Edgar Mitchell



--
----------------------------------------
Los fines no justifican los medios, porque la medida verdadera de nuestro carácter está dada por los medios que estamos dispuestos a utilizar, no por los fines que proclamamos.
----------------------------------------

“You develop an instant global consciousness, a people orientation, an intense dissatisfaction with the state of the world, and a compulsion to do something about it. From out there on the moon, international politics look so petty. You want to grab a politician by the scruff of the neck and drag him a quarter of a million miles out and say, ‘Look at that, you son of a bitch.’” — Apollo 14 astronaut Edgar Mitchell



--
----------------------------------------
Los fines no justifican los medios, porque la medida verdadera de nuestro carácter está dada por los medios que estamos dispuestos a utilizar, no por los fines que proclamamos.
----------------------------------------

“You develop an instant global consciousness, a people orientation, an intense dissatisfaction with the state of the world, and a compulsion to do something about it. From out there on the moon, international politics look so petty. You want to grab a politician by the scruff of the neck and drag him a quarter of a million miles out and say, ‘Look at that, you son of a bitch.’” — Apollo 14 astronaut Edgar Mitchell



--
----------------------------------------
Los fines no justifican los medios, porque la medida verdadera de nuestro carácter está dada por los medios que estamos dispuestos a utilizar, no por los fines que proclamamos.
----------------------------------------

“You develop an instant global consciousness, a people orientation, an intense dissatisfaction with the state of the world, and a compulsion to do something about it. From out there on the moon, international politics look so petty. You want to grab a politician by the scruff of the neck and drag him a quarter of a million miles out and say, ‘Look at that, you son of a bitch.’” — Apollo 14 astronaut Edgar Mitchell



--
Andrés Conrado Montoya
Andi Kú
andresconrado@gmail.com
http://sesentaycuatro.com
http://messier87.com
http://chiquitico.org
----------------------------------------
Los fines no justifican los medios, porque la medida verdadera de nuestro carácter está dada por los medios que estamos dispuestos a utilizar, no por los fines que proclamamos.
----------------------------------------

“You develop an instant global consciousness, a people orientation, an intense dissatisfaction with the state of the world, and a compulsion to do something about it. From out there on the moon, international politics look so petty. You want to grab a politician by the scruff of the neck and drag him a quarter of a million miles out and say, ‘Look at that, you son of a bitch.’” — Apollo 14 astronaut Edgar Mitchell