TYPO3 Core

I had the same issue. I solved it by clearing browser cookies and truncating the be_sessions table.

Hi everybody,

same issue here, but Jarvis' solution didn't work for me. Tested it with some Browsers and OSs and did the same things Lukas did.
I'm wondering if the issue is concerning Multidomain-Setups due to XSS-Defence-Approach?

Cheers,
Uwe

Jarvis' solution didn't work for me either.
I changed the PHP version from 7.2 to 7.4 and then back again to 7.2.
Now it seems to be working. I don't have an explanation for it.

Lukas Hausammann wrote in #note-3:

I changed the PHP version from 7.2 to 7.4 and then back again to 7.2.
Now it seems to be working. I don't have an explanation for it.

Changing PHP version at least "clears" OPcode cache - maybe that was the reason.

I can confirm Lukas' workaround - changing PHP-Version down- and up again worked with me. It seems to be related with how your OPcache is managed - Oliver explained why.
I my case, only Systems on Managed Servers with the hoster "Hetzner" were affected, when the option "OPcache" was checked in the PHP-Settings.
On root-Servers with the same hoster, I had no problems at all, neither on all other Systems (local DEVs as Stand-Alones or VMs) with different OS and PHP-Versions.
Obviously, there seems to be a bug in the update-procedure (and not in the 9.5.23-Core) in combination with OPcache-Flushing, since this worked like a charm with older updates.

EDIT: My assumption concerning Multidomain-Setups proved to be wrong, weanwhile.

Had the same problem, deleting the old core folder solved the problem for me.