This is a general windows error message when some application "crashes". This message was shown also when the "fast-forward-exit"-crash happened.
Do you have the latest client, 4.09 ?
You said it happened during the levelset changing. If you can, try to cause the crash again and try to find a logic to it (certain levelset change, certain amount of levelchanges etc.) so maybe then it would be easier for Wil to fix. But as we all know Wil is the man and might be able to fix it even with this information.