HTTP Error 500.19 - Internal Server Error
Absolute physical path "C:\inetpub\custerr\C:\inetpub\custerr" is not allowed in system.webServer/httpErrors section in web.config file. Use relative path instead.
Looking up and down the Internet, I could not find any useful clue at all. Out of the whim, I decided to move the mailbox that my admin account is associated with to Exchange 2013 and problem fixed!
Hopefully this is useful for some other poor folks with the same problem.
There are a lot of discussions about Exchange 2013 CU6 causing co-existence and hybrid problems but I do not this has been documented before.