I have similar problem, with Dorico Pro and BBSO Pro… We did some test together with @Ulf from Steinberg, there is some problem with the VSTAudioEngine and also with the ASIO Guard function, which is permanently active.
How many CPUs, and CPU Cores your computer has?
I would suggest you to contact @Ulf here and explain your case in details. There is a temp solution on the problem I have - VSTAudioEngine with ASIO Guard turned off, you could ask him to give you a replacement version of the Audio Engine.
If your problem is caused by the same circumstances as mine, then you will need to wait. The guys are working on it. We investigated, mainly Ulf, the problem directly on my Workstation via Team Viewer, few moths ago, so I believe it will be fixed in Dorico 4.
If you can provide more details to Ulf, it will be better for the improvement of the Audio Engine.