Just as a final follow up. In the meantime I had an extensive exchange with user Vilnai via private mail, where we finally nailed down the problem. Actually it turned out to be a bug in the audio engine that only happens when running the computer for longer than (around) 3 months without rebooting.
It was pretty tough getting to the ground of it, we had to look at so many spin dumps and log files. But the effort was worth it, as now every Dorico user will benefit from the outcome. Needless to say that a fix for this will appear in the next Dorico update.
In the meantime, the workaround is to reboot the machine; that’s enough, as the next update will definitely come within the next 3 months.
Many thanks to user Vilnai for reporting this issue and his relentless effort of providing spin dumps and feedback.