System error during lock operation. Execute diagnostics in SM12

What causes this issue?

An error occurred in the SAP lock management whilst trying to to lock
an object by calling the ENQUEUE function module.

System Response

The SAP System is unable to enter the required locks in the lock tables
managed in the ENQUEUE server. The ENQUEUE function module therefore
triggers an exception. If this exception is not handled by the
application program, the current transaction will be automatically
terminated.

How to fix this error?

The most frequent reason for the error message is that you can no
longer access the ENQUEUE server. The reason for this could be that
either the ENQUEUE server itself or the message server that creates the
link between the application server and the ENQUEUE server in a
decentral system configuration is not running.
Contact the system administrator responsible. Execute the diagnosis
function in the lock management to analyze the problem: Transaction
SM12, <LS>Extras -> Diagnosis -> Diagnosis in update task</>.
With SAP's agreement you can use the extended diagnosis functions that
are shown when you enter OK code "TEST".

Error message extract from SAP system. Copyright SAP SE.