[@robin-w Moderator 8 years, 9 months ago]

see this link

http://www.wpbeginner.com/plugins/how-to-deactivate-all-plugins-when-not-able-to-access-wp-admin/

The issue is with this plugin

bbPress Login Register Links On Forum Topic Pages

which was updated a day ago.

so don’t re-enable that one

if you go to that plugins support page, you’ll find someone has already logged that error.

You should revert to the earlier version until it is fixed