Thank you for your feedback, @mavros! – I did a complete new install yesterday night, and today my BBCSO template is working again. I think that my previous installation didn't correctly respect the fact that BBCSO must be installed in Rosetta mode. And even though I followed the guidelines on BBCSO and repaired each and every libs, I assume it wasn't picked up correctly.
As for the "double-click in the finder only starts the software": yes, I have that, too. (I can live with that at the moment.)
My new problem is: how to make the strings play portamento in Dorico? Found this:
but, being totally unfamiliar with all those MIDI channels, I don't know how to translate that to Dorico. Somewhere I read that "sustain" should be channel 64 in Cubase, so I assume it must be that in Dorico as well. But doing that does not trigger the portamento. Are you by any chance familiar with that? Are the VSL strings better at handling gliss?