Site performance, Server errors, outages and tunings

[Like Ra]

Nope. I have no idea what to fix, since I tried virtually everything. I can wait for the next release and hope that the upgrade/install scripts will fix it (it might be the database, like last time I had this issue, but I figured out where "it" was hiding)

[Tinker D]

Ok, how many cookie files are there in your system?
You should the user section, administrator section and a system or restricted section.

[Like Ra]

https://wordpress.org/support/article/cookies/

[Tinker D]

Ok, that’s good information to know.
Have you cleared or updated your operating system since you like in last?

[Like Ra]

Patched/updated you mean? Yes, of course.

[Tinker D]

I’ll bet your cookie got corrupted.
Have you tried to reset the cookie?

[Like Ra]

Have you tried to reset the cookie?

Have you tried to purge all cookies?

Many times. And changing cookie domain, and changing cookie paths (also separately for the users and the admin)...

[Tinker D]

Ever find out about your cookies ?

[Like Ra]

Guess what? After the latest upgrade of PHP (7.4.2 -> 7.4.3) it began to work. Apparently the site was hit by this bug: https://bugs.php.net/bug.php?id=79174

[Tinker D]

Really? So it was a buggy routine ?