Dorico 3.5 - issues with Mixer

The situation is as follows:
I have 2 windows open, one in Write mode, one in Play mode. Without mixer I can start playback on any of them with no problem. I open Mixer and on both of them the problem begins immediately.
I close Write window but Play and Mixer are active, the problem persist.
I close Play window, but Write and Mixer are open, all gets back to normal.
So, what do you think is happening?

Does the problem happen though if you do this?

  • Open the mixer
  • Click on the main Dorico window (in Write mode or Play Mode)
  • Press P

OK, one window only and the Mixer. In Write mode all is OK, in Play mode the problem with playback reappears. No hitting of P or space would make the playback move.

You haven’t said whether you did the important bit in step 2 to click on the Dorico window.

Yes, every time.
But a few minutes ago I have discovered something that may be helpful. I have just downloaded a simple three-instrument MIDI file I opened with Halion Sonic SE. All worked flawlessly, so either the size of the score has something to do with it, because the score that gives me problems is full orchestral piece, or the fact that all instruments are from BBC SO. However, when I changed 2 of the three downloaded instruments to BBC SO and the third to Play all still worked correctly as well.