I don't know if I can help, but I was having similar problems upgrading from 1.7x to 2.06 a few months ago. Same error sequence:
[LOGIN] Successful login for 'user'
[LOGF] Failed login with redirect
If I recall, after hours of trying things and reading source code on github I traced the the log message to a file that had been slightly modified in 2.06 (I forget which!) so I replaced 2.06 with to 2.05. Worked fine in testing, so I upgraded our production server.
However, the same problem and error message has started happening again even though I haven't upgraded. I did disable the banhammer plugin, but I don't know if those are linked. None of my users (including me) can login on a different browser. I'm a bit terrified that if they logout by accident they'll be stuck.