I've seen several other posts like this and none of them, apparently, has been resolved.
I upgraded my subs to 2.7 with no problems. Except for a white screen problem which I fixed ... plugins issue.
Feeling heady from success, I upgraded my main site. Looks gorgeous, can't login.
I deleted the plugins folder. Still couldn't login. Reuploaded it.
Cleared cache and cookies. No go.
Renamed plugins folder. Nothing.
I'm not good with code, and besides, I don't see why I should correct the code that should have been right in the first place. This has been an ongoing problem with each upgrade, and you'd think it'd be fixed by now.
This is the same scenario with Flock, Firefox, Safari, Opera, or any of the other obscure browsers I have for checking.
If this is ever fixed, I hink I'll just stay ut until someone says that there's no login problem with the latest upgrade.