No DN Launcher Queue Manager job found; yet is locked, preventing restart

What causes this issue?

The monitoring logic has detected a potential issue, which consists of
items waiting to be processed in the DN Launcher queue. Yet it is unable
to find the job that manages the queue, and a lock that will prevent a
new manager job from being started exists.

System Response

The monitor will report the manager process as "lost". Subsequent user
initiated refreshes of the monitor will continue to re-asses and will
restart the manager if the lock is released.

How to fix this error?

Note that the lock may be legitimately set if the queue manager process
is being run in an unconventional manner; for example if being debugged
or if run directly outside of the standard DN Launcher framework. The
monitor is looking for a job that it recognizes as one submitted by the
DN Launcher, and if not found, and a lock exists, will raise this
message.
The user must periodically refresh the queue to determine if progress is
actually being made; that is, the manager proces is alive and working in
an unconventional mode. If this is the case, no action needs to be
taken.
If it is observed that no progress is being made on the queue, then a
reasonable next step would be to have a technical resource review the
source of the lock in transaction SM12, and have them manually delete
the lock if they are able to determine that the process/user owning the
lock has no valid reason to be holding it.
It has been observed that this condition can occur when a user has
entered debug mode in the monitor, and has let the session time-out. In
such cases, deleting the lock in SM12 is a simple remedy.

Procedure for System Administrators

The lock object in question is /PRA/E_PN_DNL.
This lock can be deleted if it can be positively determined that the
lock is being held in error, and no other processes are active that are
executing program /PRA/R_PN_DNL_QUEUE_MGR.

Error message extract from SAP system. Copyright SAP SE.