When clicking Tools & Settings > Application Vault page in Plesk,
502 Bad Gateway error appears.
Plesk MySQL engine is mariadb-server 10.x .
MariaDB 10.x is not supported according to Plesk 12.0 Administration Guide .
Downgrade to mariadb-server 5.5 and restore databases from backup.
Additionally, you can try to upgrade to higher Plesk version, however, this scenario was not verified.