Check Computer\HKEY_USERS.DEFAULT\Network for mapped drive letters.
I had disconnected network drives that would not disconnect or delete. This must be an obscure issue as I ALL data I found on the Internet did not mention this place in the registry not once. I had to use a tool from Nirsoft to do a scan of the registry of the UNC. It found this obscure registry path. I deleted the drive letters from this path and it fixed the problem.