In the folder list, the mailbox name is blank instead of showing as "My mailbox". I have seen this before and know that somewhere in the cobwebs of my brain is the explanation but I can't conjure it so am asking if anyone can remind of the cause and the easiest cure.

You might want to take a look at your post in this thread: https://community.pmail.com/index.php?u=/topic/11870/can-this-screwed-up-install-be-repaired/1#post-55353 and proceed from there (it's about HIERARCH.PM as far as I followed the search results).

Thanks Michael. I am trying to help someone off list/off forum with a similar issue to that discussion. It reassures me that I am the right track.

Out of curiosity, what search did you perform to find that discussion? My search attempts failed to find that particular one.

I took it exactly as you wrote it down including the quotes and searched this forum (you need to do it outside of a thread): It suprisingly found close to 400 mentions, but this one was already on the first page - not exactly this particular post, but its thread close to it.

To follow up on my inquiry...

The common thread to the two problems is that the mailbox directory had somehow been added as an added mailbox. The "somehow" is unknown.
The added mailbox would not stay disconnected.
Pegasus Mail would not gracefully shutdown (lock file left in place).
Logically, a rebuild of HIERARCH.PM should have solved the problem however, if the rebuild was done following a shutdown with the added mailbox connected, the rebuilt HIERARCH contained the added mailbox.
The solution in both cases was to rebuild HIERARCH immediately after disconnecting the added mailbox.

There are always issues a developer would never think about in advance ...

I think the big question is how the home mailbox location is getting attached as an added mailbox. One individual thought it was the result of errant keystrokes but the closest I can come up with is Alt+O+A which opens the "Add mailbox..." dialog but going any further requires intent. I suppose it is possible that someone unexpectedly presented with this dialog could assume that Pegasus Mail is asking for the path to the mailbox directory. Neither individual remembers doing so.

A routine that prevents the path to Home or New from being attached as an added mailbox may be the way to prevent this.