My solution was to
- change the username in
/etc/default/plexmediaserver[edit: this did nothing, see below] - re-mount (with bind) the
Videosfolder by itself to a new root folder and - set permissions as in Chuck’s instructions.
(Following the tip by itself did not work.)
I added the following to /etc/fstab
/hdd/Videos /VideoMount none defaults,bind 0 0
This finally allowed me to add libraries at /VideoMount/Movies and /VideoMount/TVShows.
(Strangely, for that last one, TV Shows with a space no longer worked as it did on my earlier machine; I had to eliminate the space.)
I have no idea why this incantation worked and others did not.