The problem was caused by some wrong lines in the file /etc/nginx/sites-available/mainsite. First of all, it was not including the file /etc/nginx/una_params, which contains correct PHP settings. If your PHP version changes, you need to edit the file una_params, not mainsite.
This is very helpful. It seems the Ubuntu updates included a change to the PHP version. I believe this would be useful info to include somewhere in the Wiki. Thank you for your assistance in resolving.
