Sorry, I'm not able to reproduce the error anymore. I have already deleted/overwritten all corrupted savegames and started anew. This is my third attempt on my quest for killing the big Z. But now, I went back for training in between and didn't try to do all the lower levels in one go.
I encountered that error on two occasions.
Perhaps it had to with my cleric having had enough XP to gain 2 Levels. She lost some 8000 XP in the process. Then I quit UAShell as usual. Next time I launched UAShell, the save game was corrupted.
As I mentioned earlier, I couldn't even remove the corrupted character from my party. The game produced some 'save disk error'. Some effects were: strange XP values, character levels, ability scores reduced to 0, AC 0 became 60 (it seemed as if the game switched the values of AC and HP), alignment change, new race, female char became male, negative encumbrance, etc.