Loading the back-end works fine, landing on the authentication screen. Sign-in does not work, I am redirected to the same page with no error message.
You need to have:
EZP-22414, more specifically this commit
Could be a duplicate of
EZP-22665 since the main symptom is the same. On the other hand, that one was marked as master only, and the reported PHP error does not occur on 5.2, indicating a difference.
It depends on the fix for
EZP-23307 (adding the missing file). Without the missing file, login works (but you get a PHP warning about the missing file). Both issues must be fixed before either can pass QA.