My system: Nextcloud 22, Ubuntu 18.04, Plesk 18.0.40
If i click on the admin site to System - there comes that error message.
The server was unable to complete your request.
If this happens again, please send the technical details below to the server administrator.
More details can be found in the server log.
there should be a error.log as well. serverinfo-app calls a lot of information from the system, that can trigger errors (should only happen when you open the admin-page).
In the apache error log is that standing, after i click in the nextcloud / admin / system:
[Sun Dec 19 13:01:34.641548 2021] [ssl:warn] [pid 21580:tid 139720537380608] (22)Invalid argument: AH02026: Failed to acquire SSL session cache lock
[Sun Dec 19 13:01:34.641618 2021] [ssl:warn] [pid 21580:tid 139720537380608] (22)Invalid argument: AH02027: Failed to release SSL session cache lock