macOS: Dorico 3.5 does not start / "Audio engine: Waiting for connection"

Problem

Dorico 3.5 hangs on launch. In the start screen, the following message is being displayed indefinitely:

"Audio engine: Waiting for connection"

Background

This problem can occur if you have installed an incompatible HALion Sonic SE version, for example by trying out Dorico 4 on an unsupported macOS version.

Solution

If this is the case, "downdating" the existing HALion Sonic SE installation to HALion Sonic SE 3.4.40 will solve the issue.

Restart your computer to make sure no Dorico process is still running.
Download and run this HALion Sonic SE uninstaller.
Download and reinstall HALion Sonic SE 3.4.40.

Now it should be possible to start Dorico 3.5 again.