NZyan

I think I found the issue, it's related to this one:
My provider switched to mySQL strict mode during the mySQL update.
This triggered an error during the creation of a new account.
The table ipb_core_pfields_content had two fields for Stop Forum Spam (not sure if this is still installed), one of them without default value.
Looks as if the SQL statement fixes the issue:
ALTER TABLE `ipb_core_pfields_content` CHANGE `sfsNextCheck` `sfsNextCheck` INT NOT NULL DEFAULT '0'; @Marc Stridgen Does this explain the multiple accounts?
Andreas

Update:
With help from the IPB support I found out that the problem was on Bonsai’s (ElasticSearch provider) side:
Bonsai quietly stopped support for the „update by query“ command for their smaller packages.
Took me weeks to find out, annoyed my members.
Their suggestion is to upgrade to their business packages with price tags starting at $700.
My summary: Do not use Bonsai for Invision.
Andreas

For now: Yes.
Hopefully this will not again, especially not on a day when I don't visit the admin panel.
Andreas

OK, solved.
Solution was to delete not only the cache but also „Cookies, Databases and Local Storage“ (translated from German „Cookies, Datenbanken und Lokaler Speicher“).
This setting can be found in Safari via Settings > Data Protection > Manage Website Data