I have released version 2.5.11 to fix this issue specifically.
I can't say with 100% certainty what the issue was, but after testing on Win XP, Vista, and Windows 7, it appears the issue only happened on Windows 7. It was not the auto-save technically that was causing the issue, but anything calling from the clock thread into the main program. Turning on the auto-save just increased the frequency of the issue occurring. In my long running tests, the error occurred about 20 times per 10,000 auto-saves, or about once out of every 7,000 possible times it could occur. That's pretty infrequent, so I'm fairly surprised you guys saw it more than once or twice in any tournament. I just put in some good error catching code (where none was ever needed before), and the error is just ignored and the tournament moves on.
I was able to reproduce this problem pretty easily on Windows 7, so I think I got it. But I would certainly appreciate feedback from you who have also seen it.
Thanks again.