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For whatever reason, using Linux, I can no longer play the game past Hotfix 15 was the last version that I know for sure worked (there may be newer, but I only have that save game available). I have version and that works perfectly, but I can't even download that version any more.

can you make more of the older versions available? Even the oldest version I can get access to won't work for me.
Gog only stores the last 5... you would have to get Larian to agree or find a torrent someone has iligally posted to work around that