This may sound odd, but I installed a separate copy of DNN, and again got this error. This time around, however, the event sequence leading to disaster was different.
As I said, I am developing custom modules for DNN. During that process, I had DNN running through VS. During debugging of my module, I found a mistake in one of the DLLs I had created & put in /bin folder for my module to use it.
I corrected that error from another instance of VS, recompiled the DLL & put it again in DNN /bin folder (remember, DNN was running).
Then, I got a "Could not unload AppDomain exception." I stopped debugging & then restarted, but to get the same "Upgrade" page being displayed. I think, I might have done this last time also, and modified web.config after I got this error, and so thought this was due to that modification.
So, what now??? Is there any way to restore these installations to normal?? (There is no question of Permission errors, as DNN root has FullControl for everyone)