I had problems updating Moodle since June 2019. All my Moodle updates under Oracle Enterprise Linux since 2017 had worked like a charm, but after June things were in a mess. Immediately after saving the fresh ./moodel/config.php to the web servers file system and hitting the "Proceed" button, the web brower would tell me: "error/An error occurred whilst communicating with the server" eller in Norwegian "error/Det oppstod en feil i kommunikasjonen med serveren." -
Turning on debugging under php (set display_errors = On in the /etc/php.ini and restart the server - service httpd restart) would not result in any error message. The use of Norwegian was very strange since my primary server environment is English only. Stranger still, when my test installation on the same server failed in this manner, the production server on the same server would fail, too. Sometimes a reboot of my webserver would fix the problem, sometimes it would not. Turning SeLinux on or off (setenforce 0) would not help either.
After very careful testing I thought I had found a workaround for upgrading from 3.7.2 to 3.7.3. Today, after everything had been running for 10 hours, the server crashed - again. I had the inspiration to try the Moodle app on my Android phone - my Mood site was still running. Then I logged on my site using a different web browser then Microsoft Edge - Google Chrome - and I got in. I then hit upon the idea that this was an instability in Microsoft Edge and I restarted Edge. I came in! This finally explains while some error messages were in Norwegian (when I use a hospital-owned Win-10-machine) and othelrs in English (when I use my private laptop).
Moodlers - beware! When strange things happen on the server, try another web browser.