All,
I had the same problem. In my case, it turns out the culprit was a plugin called “Enforce www. Preference“. Found it this way:
1) Through FTP, Renamed \plugins folder to \NOplugins (simple way to deactivate all of them without deleting files).
2) Logged in normally; worked fine. Goto Plugins admin area. (Note the error message saying all of them have been uninstalled.)
3) Through FTP, returned the plugins folder to its original/correct name.
4) Re-activated each plugin one-at-a-time, logging off and on after each activation.
When I activated the “Enforce www. Preference” plugin, the login loop problem returned. So through FTP I deleted that plugin, and all is fine!
Good luck!
