Alain Raap

Did you clear the Moodle caches after the upgrade? Sometimes this can help with these kind of errors. In Admin -> Development you can find a link to the admin/purgecaches.php script. We also had several of these problems after upgrading and this solution helped most of the time. Another reason can be that old code of a previous version is still present in your Moodle folder. This depends on how you upgrade your Moodle site, which is explained here https://docs.moodle.org/39/en/Upgrading

Nadav, thanks for sharing and interesting to read the chat, I only missed the photo's that also gives interesting information (about architecture or technical information).

Kelvin, max_input_time 600 is 10 minutes, for production environments 60 is advised (1 minute) as you can read in the php.ini file

I found a blog of Tomasz Muras about session locking in Moodle that might be interesting:

https://muras.eu/2020/04/22/Session-locking-in-Moodle-part1/
https://muras.eu/2020/04/25/Session-locking-in-Moodle-part2/
https://muras.eu/2020/05/17/Session-locking-in-Moodle-part3/

We use Redis caching for both application and session, about 700 concurrent users, this works without problems.

The number of backups each night is the usual number again we had before the migration. So I think the upgrade caused the large amount the first time
the automated backup task ran.