I experienced exactly the same problem when trying to upgrade forum (...rebuilt database and did all the ?flush=1 things but without success...). Had to roll back to previous forum version, and then admin login page behaved as expected.
Is there some OpenId thing that has to be set up before upgrading forum!?
I couldnt login to admin but I tried the solution at http://www.silverstripe.com/website-authors-forum/flat/17849 and it worked.
Initially I got an "Out of memory" error but I was able to login after that.