I have the same issue - plugins not showing. Pressing reload does not work. Fresh install v 3.1.1. PHP version 7.2.33, mySQL version 5.7.34, apache version 2.4.46.
The install script is by Softaculous. On install, it wrote the wrong mySQL version (5.5) in the config.php file in the Bundles/corebundle folder (line 1124). I corrected it and then I cleared the cache (note that you need to update your documentation as it does not list the correct location of the cache - now in the var folder, not the apps folder!). Still not working.
This same error seems to have been reported several times and still there is no fix.
UPDATE _ FIXED.
I found that I had to also amend the mySQL version from 5.5 to my real version (5.7.34) in the app/comfig/local.php file on line 36…
However, this error should not have occurred in the first place…
I ran sudo -u www-data php /var/www/html/mautic/bin/console cache:clear to clear the cache but then it looks like it killed Mautic. I figured since it’s already dead, I also tried rm -rf mautic/var/cache/*. Didn’t look like it did anything. Nothing comes up when I go to the server now. Just says the site can’t be reached.
Also, tried updating Mautic thinking maybe it might restore some accidentally deleted files. I think that brought back the /prod folder in /cache.
Any other troubleshooting tips to see why I can’t access Mautic from the browser anymore?
Is there such a thing as a force “restart” of Mautic?
Or how do I restore the cache like @javigdiaz mentioned without being able to access Mautic from the browser?