Right after running Plesk Update, the following error appears trying to access Plesk:
PLESK_INFO: Server Error
Web server received an invalid response while acting as a gateway or proxy server.
The error disappears when Plesk updates are completely installed.
It is an expected behavior. Plesk services (sw-cp-server, sw-engine) are being restarted during Plesk updates installation.
Wait till Plesk updates are completely installed.