Ctrl+T always works.
The + button only works if I've pressed Ctrl+T on that tab once. And then only that tab's + button works. All the other tabs, no go.
No luck.
Ah. That lead to some clues. Seems when turning off the Event.Folder_CloseTabSiblings.js
(v 11.6 tbone) makes + button work. Script description: "Close tab siblings (tabs with same location)". Guess I'll have to mention this in the script's forum thread. Case closed as far as DOpus main dev goes. Edit: Reported my issue on the script's thread.