I think that you'll need to provide a bit more info to get meaningful solutions.
- Do you have server logs?
- Can you debug through the app as the error occurs?
- Does the error occur at a certain time of day, or after a certain regular action?
- Does your app attempt to write to a file that may not be accessible?
- is it possible that you are experiencing memory issues?
the list could go on, best to do some more investigation and if a more specific issue comes to light edit your question with the extra detail.
AFTER EDIT:
From the extra detail you've provided I wouldn't jump to memory as an issue, in signing out and back in the user is refreshing their session so everything is reset. If you are not seeing anything in your logs you'll need to look at your exception/error handling.
You just haven't provided enough info yet for us to work out the root issue, let alone suggest a solution. That's what you're seeing from all the answers here thus far. Find the event log info and there should be something there to help you, or at least something more to post here.