>Gut reaction is that it’s a clash between plugins – the usual solution for that is to remove the likely offenders and then reinstall >one by one.
Good suggestion. I’m quite familiar with that troubleshooting method. I’ve used it many times. This was actually one of the first things I tried this morning (out of a long list of many items). Unfortunately, it didn’t make any difference. I got the error even with all plugins deactivated.
>It could also be as simple as a cookie error – try logging in on a different browser or via a private/incognito window.
I actually cleared all cookies and cache on this machine. However, I haven’t tried logging in from another machine. Just tried it. Same error. I’m gonna upload a file, force-upgrade.php, that someone gave me in the OP FB group and try that.