AIR Mini Grand: no 'long' sound, just short notes

[dvdr]

Just upgraded from Pro Tools free to 12.5.2.
When I first created an Instrument track, I could assign Mini Grand and play on its virtual keyboard via the mouse. No problems, held notes were long, I could change the sounds and settings.
Shortly after, Mini Grand only produced super-short sounds (pitch as per the key I hit), but no real, long sound - neither via the virtual keyboard on the screen nor with my midi-keyboard.
Exchanging Mini Grand with Xpand!2 produced perfect results, so my keyboard or any other midi settings seem not to be the culprit.
I reinstalled the the Air Instrument collection, but no change. I rebooted everything, no change. I created a new session from scratch: no change

Mini Grand will only produce a super short sound, and that's it....

I hope, someone will be able to help me.

Thanks!

protools.ideascale.com/a/dtd/Mini-Grand-plays-only-the-attack-of-each-note-macOS-Sierra/262663-3779
avid.force.com/pkb/articles/troubleshooting/PT-223601