wp-login reauth=1 login fail - redirect loop

Has anyone come across this recently? I have seen it on a few of my sites and Siteground support couldn't offer any help at all.

I tried every suggested solution found online - different machine and browser, default .htaccess, disable plugins, clear cache/cookies, delete wp_usermeta session_token etc. etc.

Discovered it was PHP 7.4 in my case, and switching back to 7.2 or 7.3 fixes it.