Howard Miller

We spend a LOT of person-hours testing prospective upgrades extensively. While you may be able to trust core Moodle, it's very rare for anybody to run "vanilla" Moodle. Not all optional plugins are so well tested and we also have all manner of integrations with potential to come unglued.

So, yeh, Moodle sites can be surprisingly complex and interlinked. For anything beyond the trivial, please, please test your upgrades somewhere that doesn't matter.

I don't want to reply by saying, "that's nonsense, don't listen to this guy".

However, Moodle is a complex system. The range of server configurations that it can be installed on will have almost infinite permutations and is almost entirely unpredictable. User skill is also higly variable.

So, yes, things can and do go wrong. That's what these forums are here for. We will do our best to help.

But to say, "do not upgrade" is, I'm afraid, just crazy talk.  I also note that you are a Softaculous user. Softaculous is nothing whatever to do with Moodle. If that screws up then, please, complain to them, not us. 

Checking that you have....

$CFG->dblibrary = 'native';

set?

From config-dist.php

$CFG->dbtype    = 'pgsql';      // 'pgsql', 'mariadb', 'mysqli', 'auroramysql', 'sqlsrv' or 'oci'

Note, the comment. It's 'mysqli', NOT 'mysql'

No - you either have editing capabilities or you do not.

A Teacher that only has read-only access is not a Teacher - that's the 'Non-editing Teacher' role... which sounds like what you want.