Howard Miller

...and that was a specific block (the HTML block) rather than *any* block... which sounds like what you mean.

The only thing I've got... could something be bodged together with the Generico filter? Possibly to hide blocks based on profile fields? It's a long shot...

Have you any optional plugins that may be related? I guess that they would most likely be authentication, enrolment or local plugins (in that order).

I'm struggling to think of anything in core Moodle that would do this.

You may also be connected to some external auth system (e.g. OAuth) that *could* have something to do with it. Although, I know very little about these modules.

I very much doubt that is possible. That will be the same capability that allows them to edit other user's profiles.

Whatever else you do... take a complete backup of your database data before the upgrade is performed. Then it IS reversible.

What sort of authentication is your site using? Are these just manual accounts or something else?

If it was working fine and then it suddenly wasn't then *something* changed. Even if it doesn't seem releated. It would be a good idea to find out what.

Moodle 3.9 has been completely unsupported for well over a year, so that isn't a great place to be.