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.

edit:
You need to have:

[DebugSettings]
DebugRedirection=disabled

Regression from EZP-22414, more specifically this commit
https://github.com/ezsystems/ezpublish-legacy-ee/commit/841690362de5e35030402d41a7a1ef4394f9fcfa

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.