Alan Bowen schrieb am 29.04.2021 um 18:29:
> With  ConTeXt  ver: 2021.04.28 18:39 LMTX  fmt: 2021.4.29  int, I am 
> now getting the following error on compiling with files that were OK 
> previously. Is there something that I need to change?

The error is caused by the fallback font for math but you have create a 
minimal example because I'm unable to reproduce it (copy all font 
settings to a new document and run ConTeXt).

Can you also check if the font files are found by ConTeXt:

         mtxrun --script fonts --list --all stixtwotext*

Wolfgang