Well, at least you now know phpmyadmin is not the root problem
It does sound like a problem with /etc/nginx/sites-available/22222
Was the domain added as a php7 site? ('ee site info domain' will tell you - I'm just thinking the fixes you referenced above were php7 specific)
After you make any changes I'd also run
sudo nginx -t
which just tests the nginx config for any errors before you put changes live.