[SOLVED] moOde 9.0.5 and 9.0.7: Adding radio station to playlist does not work

[the_bertrum]

@TheOldPresbyope , @Tim Curtis , @Nutul I just sent you via PM a movie where I explain how to reproduce the issue, but also how you can workaround it.
As mentioned earlier, I bet on a playlist file locking issue, caused by MPD locking the file when scanning the PL folder.
The reproducer scenario works consistent on my 3 PIs.

Just so I'm sure I've got the case straight in my head, you get this error when:

A stored playlist is loaded into the queue, you start to play that playlist, then you try to re-add to the playlist a station that is already on the playlist?

Or


A stored playlist is loaded into the queue, you start to play that playlist, then you try to add a new station to the playlist?

And you get the error when using the three dot on a station and selecting "add to playlist" but not when adding a station to the queue then saving the queue as a playlist?