[1.7.6.7] Backend login silently fails, re-displays login form

[AJ1234]

Hey.

As in title; when I successfully login, nothing happens -- I am being shown the login form as if I had just opened the page. Any ideas? Thanks.

Points:

• An update from 1.7.6.5 to 1.7.6.7 was one of the things that triggered it, but it happened before, too (triggered by something unknown), which I avoided by restoring a server backup.
• The login itself succeeds; giving wrong data displays relevant message.
• Clearing (renaming) /var/cache/prod doesn't help.
• Browser choice/config seems irrelevant.