As the title suggests, the programme does not survive past the splash screen if JACK Audio has been installed and registered. This problem was not present in MuseScore 2. It is not certain whether this also happens on OSs other than Windows.

How to Reproduce the Regression

• Install 64bit JACK2 for Windows
• As per the installation instructions, execute regsvr32 "C:\Program Files (x86)\Jack\64bits\JackRouter.dll" with Command Prompt as administrator
• Attempt to open MuseScore 3

The programme shows the splash screen before crashing, without entering the main programme window.

Choosing "Debug" reveals that it is libjack64.dll causing the issue, but does not yield any further details.

I was able to reproduce this on a clean virtual machine running Windows 10 as well.
MuseScore 2.3.2 runs perfectly fine in these circumstances.

"Workarounds"

So far, I have not discovered any real workarounds to the regression, but there are ways to restore MuseScore's functionality:

• Unregistering JackRouter.dll with regsvr32 /u "C:\Program Files (x86)\Jack\64bits\JackRouter.dll" allows MuseScore 3 to work normally again, but obviously renders JACK Audio functionless
• Starting MuseScore3.exe with the -s flag, which disable's MuseScore's synthesizer, allows the programme to run without needing to modify JACK Audio, but also removes any audio output.
• Installing the 32 bit version of JACK Audio does not interfere with MuseScore, but it has been impossible to make JACK Audio detect any midi output from MuseScore, making it frustratingly non-functional