A note for those of you using log4net:
I recently upgraded from framework 2.0 to 3.5 and all of a sudden my web app shut down unexpectedly. Turned out that it was the location of the log directory (which was under the application itself ..bin/logs) that caused the issue.
Moved the log directory out of the application and now it's running nicely again :)
Please 'Mark as Answer' if my reply helped you.