Unknown technical problem with system &1 occurred. Check TMS Alerts

What causes this issue?

System &v1& encountered technical problems. It is currently not possible
to communicate with system &v1&.

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

Check the TMS Alerts for system &v1&. To do this, call the Transport
Management System (transaction STMS) in the current system and choose
"Monitor -> TMS Alerts -> TMS Alert Viewer".

Error message extract from SAP system. Copyright SAP SE.