System error detected during (un)lock procedure: call &3, cust.&2, user&1

What causes this issue?

Lock Management detected an error when it was trying to lock or unlock
an object.

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?

Perform the function again. If it is more than just a temporary error,
check the entries in Lock Management with the transaction
<DS:TRAN.SM12>Select lock entries</>.

Error message extract from SAP system. Copyright SAP SE.