What is the issue? Well...iMovie opens and immediately crashes whenever my MacBook Pro is connected to an external monitor...
Re-opening iMovie doesn't help, it just crashes over and over, as long as the external monitor is connected. Running it without an external monitor works fine.
I did manage to find a way of making it work with an external monitor however! Here are two videos, the first showing iMovie crashing when launched while connected to an external monitor. The second shows it working with my 'workaround'.
The workaround is quite simple...I just open my MacBook Pro so it's screen is active as well. When this is done, iMovie launches fine and I can then close my laptop again and keep working as usual.
It's a bit of an annoying workaround, but it works!