Sudden issue logging into Admin area

[Brewhaus]

Everything has been running smoothly for some time, but we attempted to perform an upgrade to 7.0.2 today. Unfortunately, we had to revert back to 7.0 due to some errors, and have moved back to our pre-upgrade DB and pre-upgrade web folder. However, now when we try to log into the Admin interface we get a runtime error. I cannot find the actual error, because as soon as we turn Custom Errors off in the web.config file we are able to log in successfully. Turning the Custom Errors back on does not immediately cause an error, but if we wait 5-10 minutes and then try to log in, the error come back up.

Does anyone have any suggestions?

[Brewhaus]

It appears that this may have been a permissions issue (although I am unsure why changing the Custom Errors would jog things). Reverting to copies did not carry all of the folder permissions forward for the Network Service user.