I had a similar problem here. I upgraded to 4.5 the other day and it went almost flawlessly. Today I tried to upgrade to 4.5.1, and AD authentication stopped. I logged in with a superuser account and found a similar error as listed above. I re-copied all the files from the upgrade install, but that didn't work.
So I searched the forums and found this thread. I checked the web.config and found it had more entries than my previous one (as listed above), so I transferred my custom settings to the new web.config, verified all the DLLs were in place and tried again. Then I got a different error (of course I forgot to write it down), so I restored my backed up version of DNN 4.5 to get things working for now.
Is there that many changes to the web.config in 4.5.1? I'm going to have to go through it line-by-line to find out what I'm missing.