Sys.WebForms.PageRequestManagerServerErrorException 12031
If you're getting that from an updatePanel, set EnablePartialRendering to false in the ScriptManager for the page, and then it should give you the actual error.
Also, if it only happens occasionally, I've found that it could be a viewstate problem, especially when the page goes a long time (20mins or so) between refreshes.
Otherwise, try some try/catch blocks. Those are some easy methods.
Hope that helps!
It's a viewstate problem, but not related with time but with size. Try playing with maxRequestLength in your web.config.
I've got this error in UpdatePanel with autopostback Dropdown after big delay (>20 min) between change dropdown selection.
Try increase session timeout in web.cofig. For example:
<sessionState mode="InProc" cookieless="true" timeout="720"/>;