Thanks.

After some work on it, following your advice, I reduce the compilation problem to several "wrong" files.
One was trivialy fixed because I did not close one environment, but other don't. Please help:

The error is here and I attach file.

modules         > 'pgfmath' is not found
structure       > sectioning > section @ level 3 : 0.0.1 -> Problemes de fraccions
structure       > sectioning > subject @ level 3 : 0.0.1 -> Preguntes
tex error       > tex error on line 25 in file ./dev.tex: \textfont0 is undefined in ordinal, font id 0, character 63)

<line 3.25>
    \startexercici[reference=exercici:problemes-de-fraccions-1, title={oli}] Calculeu quina part del dipòsit queda buida, sabent que l'aigua ocupa $2/5$
     del recipient i l'oli, $1/4$ (vegeu figura~\in[fig:oli-aigua-diposit]).

15     \usetikzlibrary[trees]
16     \usetikzlibrary[calc, intersections, through, backgrounds, arrows, shapes.geometric, fadings, decorations.pathreplacing, shadings, shapes.geometric, patterns, fit, positioning, shapes.symbols, chains]
17     \usemodule[pgfmath]
18     %\usemodule[filter]
19
20
21     \startsection[title={Problemes de fraccions}, reference={seccio:problemes-de-fraccions}]
22
23     \subject{Preguntes}
24
25 >>  \startexercici[reference=exercici:problemes-de-fraccions-1, title={oli}] Calculeu quina part del dipòsit queda buida, sabent que l'aigua ocupa $2/5$ del recipient i l'oli, $1/4$ (vegeu figura~\in[fig:oli-aigua-diposit]).
26
27     \startplacefigure[reference=fig:oli-aigua-diposit, location=force, title={Dipòsit omplit amb aigua i oli}]
28     \bTABLE[frame=off,align={middle,lohi},width=fit]
29       \bTR
30         \bTD
31      \starttikzpicture[scale=0.75]
32      % recipient
33      \draw (0,0) -- (0,10) -- (2,10) -- (2,0) -- cycle;
34
35      % aigua
Somewhere in the math formula just ended, you used the stated character from an
undefined font family. For example, plain TeX doesn't allow \it or \sl in
subscripts. Proceed, and I'll try to forget that I needed that character.
mtx-context     | fatal error: return code: 1


Thanks in advance,

On Tue, 15 Aug 2023 21:44:53 +0200
Wolfgang Schuster <wolfgang.schuster.lists@gmail.com> ha escrit:

> Xavier B. schrieb am 15.08.2023 um 21:36:
> > On Tue, 15 Aug 2023 21:13:54 +0200
> > Wolfgang Schuster <wolfgang.schuster.lists@gmail.com> ha escrit:
> >
> >> There is also something wrong with the structure of your document when
> >> need \page[yes] after each file.
> > Why is it trouble?
> 
> When you start each subfile with \section or \subject you can use 
> \setuphead to force a pagebreak at the start of each new section and not 
> with a \page command in the document.
> 
> Wolfgang
>