I couldn’t connect to my Plesk (Plesk Obsidian 18.0.26) panel after upgrading it. It seems that he plesk panel needs these two: services sw-cp-server and sw-engine to create the web pages to use plesk panel. Sometimes these services are stopped and/or failed to restart after an update. If these two services aren’t running it will result in a 502 bad gateway error page.
To start the services you first need to login to your server using ssh, when connected to the server copy+paste the following lines:
service sw-engine start
service sw-cp-server start
After you started the services you can close the ssh connection with your server and you should be able to login to your plesk panel.