Hello, looks like you didn't setup Plesk for remote databases. I hope this article helps you (Plesk for Linux) Plesk Installation Without the Local Database
Hello, it's difficult to determine the exact cause of the issue. I recommend checking the Access Control List (ACL) settings in your network, or reviewing similar security configurations in your data center.
Hello, I guess this article could help you https://support.plesk.com/hc/en-us/articles/19431741403671-Plesk-Monitoring-thresholds-cannot-be-updated-Request-failed-with-status-code-500
Also, please clear browser's cache.
P.S There is no newer version of plesk-json-backend-datasource
Hello, yes, PPPM-15094 introduced those changes (proxy_set_header Host $ip_default_host;) in the nginx configs. But it's unclear for me what problem do you have?
Hello, could you provide database logs as well?
Also, it looks like the table 'session' is corrupted.
In this case, you can recover this table manually InnoDB Recovery Modes
BR, Aleksei