Synfire Pro 2 stuck on starting audio engine/starup splash screen

If on Windows, check if any anti-virus tool blocks local network communication.

Also check if the Audio Engine is already started: Look for it on the task bar.

You can install v2 over an existing v1 installation. No need to uninstall first.

When you redeem the license code with License Manager, it will automatically convert the old v1 license to v2. Only thing you need to do is drop that license in License Manager on one of your connected iLoks to move it there.

Of course you need a v1 license in your PACE account that can be upgraded.

You can install v2 over an existing v1 installation. No need to uninstall first.

Except that it doesn't work, Synfire V2 then started with a generic error about authentication (even though contrary to @wozitcool I already surrendered the old license and installed the new one), so I needed to uninstall V2, which if you try to do via the control panel gives you more errors, so I had to manage to guess that you need to uninstall V2 via the downloaded setup zip, and then I could successfully re-install Synfire V2.
Didn't bother to make screenshots because I was annoyed.

That's what I love about Windows. We tested this upgrade/migration procedure a hundred times on three different computers, with Pro v1 or Express v1 or enirely virgin, and everything went fine. Only when released to the public, things start to go wrong ;-)

We tested this upgrade/migration procedure a hundred times

I had read about that. That's why I was annoyed.
Similar things happened with uninistalling the demo of V1 as I had reported in the bug thread.
No idea for the cause, I didn't even changed the default path or things like that.

You should delete all remnants of v1.

Rename any folders 'Synfire Pro' to 'Synfire' and delete SynfirePro.exe if it's still there. That should suffice. Actually the upgrade/migration should have already done that.