Which graphics driver are you using? That environment variable is not universal. However if this game really has the extension string bug, then I'd think it's better to patch the binary. However, it seems different from the Quake binaries..
MESA_EXTENSION_MAX_YEAR is correct for the GPU & drivers I'm using (AMD RX480 using the open drivers. Kernel 5.0.0-20 & Mesa 19.1).
EDIT: please try replace ref_gl.dll with this version: http://guu.fi/g/sof/ref_gl.dll
(I doubt it helps, but it's worth a try)
I tried some of those as well, looks like GOG already use a patched version though. The one you linked to does not make any difference.
Hmm, our software is similar enough that I suspect this could be an issue with the game trying to use a display mode that your monitor does not support (or that is somehow disabled in software). (I'm also using RX480 with open drivers, MESA 18.3.6, kernel 5.0.9-200)
I could not figure out a simple way to stop the game from mode switching. It's possible to disable fullscreen mode but the game still does a mode switch at startup before reverting back to the desktop resolution.
EDIT: Does it give any more detail preceding the final error message?