Document &1 currently being processed by user &2

What causes this issue?

You attempted to issue a document &V1& output. This document is
currently being processed by another user or you yourself, however, and
is therefore blocked for further processing.

System Response

If you execute the message output in the foreground, the system cancels
the edit process. If you start the message output in the background,
the edit process of the current output is cancelled, but other, future
outputs are processed normally.

How to fix this error?

Try to restart the editing process for document &v1& later. If the
document stays blocked for a relatively long time, inform your system
administrator.

Error message extract from SAP system. Copyright SAP SE.