Same here. It compiles well when the math font definition is commentend.
The error seems to be there:

\setfontid


<to be read again>

\3>mainface-12pt-mm-mr--1

<macro> \font_helpers_low_level_define_compact ...ec \empty \let
\lastrawfontcall \relax \mutable \letcsname #2\endcsname \relax \else \edef
\somefontspec {at \number \ETC.

<macro> \font_helpers_trigger_define ...\def \v_font_size_relative {#1}\def
\v_font_size_absolute {#2}\font_helpers_low_level_define
{#3}\v_font_identifier_class

\csname \v_font_identifier_class \endcsname \setfalse \c_font_auto_size
\ifskipfontcharacteristics \else \setfontcharacter...


<macro> \m_font_class_direction ...fcsname \??fontinstanceready \fontclass
-\fontbody -\s!mm -\fontfamily \endcsname \settrue \c_font_auto_size
\lastnamedcs

\else \font_helpers_set_math_family_b \fi


<macro> \font_helpers_set_math_family_indeed_compact
..._family_set_scales_compact \let \mathsizesuffix \mathtextsuffix \let
\fontface \!!plusone \font_helpers_set_math_family_a

\textfont #1\font \font_helpers_set_math_family_a \textfont #1\font
\scriptfont #1\font \scriptscriptfont #1\font \let \ma...


<macro> \font_helpers_set_math_family ...!mm -#2-\fontsize -1\endcsname
\font_helpers_preset_math_family_indeed #1#2\else
\font_helpers_set_math_family_indeed #1#2

\fi \else \font_helpers_set_math_family_indeed #1#2\fi

Jairo

El vie, 29 de ene. de 2021 a la(s) 15:59, Pablo Rodriguez (oinos@gmx.es)
escribió:

> Dear list,
>
> this sample cannot be compiled with latest LMTX (2021.01.29 19:44).
>
>   \enableexperiments[fonts.compact]
>
>   \definefontfamily
>       [mainface]
>       [rm]
>       [TeX Gyre Pagella]
>
>   \definefontfamily
>       [mainface]
>       [mm]
>       [TeX Gyre Pagella Math]
>
>   \setupbodyfont
>       [mainface, 12pt]
>
>   \starttext
>     \input zapf
>   \stoptext
>
> I get the following error message:
>
>   I was looking for a control sequence whose current meaning
>   has been defined by \font or a valid font id number.
>   mtx-context     | fatal error: return code: 256
>
> Previous latest from compiles the sample perfectly fine.
>
> Could anyone confirm this bug?
>
> Mamy thanks for your help,
>
> Pablo
> --
> http://www.ousia.tk
>
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