Properly logged in and not a cookie issue. Running multisite.
Everything was fine until the latest bbPress update. I checked the box to allow all multisite users to post to the forum. The problem didn't occur immediately after that or even the next day.
The apparent trigger was today when logged in and able to use the WP admin as normal, I navigated to Settings > Forums and it redirected me to the login page. I haven't been able to get back in since.
Checking my error_log the same error is repeating over and over:
PHP Warning: Illegal offset type in isset or empty in /wp-includes/post.php on line 840
Line 840 and 10 lines above and below at least in TextWrangler is just a programmer comment so I don't know.