Now I cannot open the DEVONthink help anymore:
In DEVONthink Pro Office I go to the Help menu and click DEVONthink Pro Office Help. The Help window opens, but it ends up having the title Mac Help and shows only the text “The selected content is currently unavailable”.
How to trouble shoot this?
ps: I don’t know when this problem started, because I do not go to the Help that often - normally I consult the ePub.
Does it work trying it again later?
Yep, it works now! You are a magician, Jim! 
But why on earth didn’t it work before? Any hunch?
Criss would have to respond on this one.
The help system of macOS can’t find the contents if they were not yet indexed, this happens especially after (re-)installing or updating the app and launching it for the first time.
Thanks Criss. Good to know.
How long does this re-indexing typically take? And is there a way to force a manual re-indexing?
Usually within few seconds/minutes. A reboot might help too.