On 18 Sep 2020, at 11:43, Thomas A. Schmitz wrote:

> Then tell us a bit more about your system. What is the output of
>
> which -a context

     /Library/TeX/texbin/context

which is an alias of

     /Library/TeX/texbin/context@ -> 
../../texmf-dist/scripts/context/stubs/unix/context

and texbin is

     /Library/TeX/texbin@ -> Distributions/Programs/texbin
     /Library/TeX/Distributions/Programs/texbin@ -> 
../.DefaultTeX/Contents/Programs/texbin
     /Library/TeX/Distributions/.DefaultTeX/Contents@ -> 
../TeXLive-2020.texdist/Contents

> for example? Is
>
> /var/folders/
>
> the normal path where mactex would expect a format file? I use a Mac, 
> but I have vanilla texlive installed; I never quite understood where 
> mactex was any better, so I can’t test on my system.

The directory name luatex-cache suggests it's used for caching. I 
interpret the output like „something happens during the generation of 
the format file and therefore the file never gets generated“. I don't 
know much about the internals.

I don't really have any reason for using MacTeX over texlive; I was once 
told that this is the thing I want to use on a Mac when I was a student, 
and have used it ever since.
I could switch over and try texlive if nothing else helps, though I 
think it would be good to pin down the cause of this error and tell the 
MacTeX folks about it.

Regards,
Felix