https://bugs.winehq.org/show_bug.cgi?id=52974
--- Comment #21 from OldCastle oldcastlehq@protonmail.com --- Ah...it's working. The issue was that my Mulab is outside of the WINEPREFIX. I have the default prefix /home/oldcastle/.wine, but for MuLab I have it in /home/oldcastle/Applications/MuLab.
After I moved it to /home/oldcastle/.wine/drive_c/Program Files/MuLab it started to work.
One question. Is this fix future-proof? I'm asking because MuLab 10 is around the corner.