Import queue &1 locked by user &2 in system &3

What causes this issue?

Import queue &1 is locked by user &2 in system &3.

System Response

The function terminates.

How to fix this error?

If you are sure that this user is not working on the import queue, you
can unlock it as follows:
Confirm the error message in the import queue and lift the lock in the
next screen.
If the error is in the TMS worklist, you can use the import queue to
lift the lock:
Call Transaction <ZH>STMS</>.
Choose <LS>Overview</> -> <LS>Imports</>. The import overview appears.
Position the cursor on system &V1&.
Choose <LS>Import queue</> -> <LS>Display</>. The import queue of this
system appears.
Choose <LS>Queue</> -> <LS>Adjust</>. The same error message appears.
Confirm it. The following dialog box asks you whether you want to lift
the lock.
Return to the TMS worklist and continue processing the transport
proposals.

Procedure for System Administrators

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

Error message extract from SAP system. Copyright SAP SE.