Since July.
Who said anything about ‘magic’ that’s a bit condescending and arrogant.
Im aware files can get corrupted, but I assure you nothing was added or taken off my Mac while I was in rehearsals (didn’t even have wifi). If something changed it was nothing I instigated.
I did, and I say it quite often,
It is simply a shorthand way to say that if anything stops working or behaves differently then something changed. Could be a hardware fault, perhaps a corrupt drive, or a RAM problem, or a cable, corrupted data, or a software change (security update, third party app that changed a DLL, a plugin update, an antivirus update that decided that GP was malicious and blocked it).
We’ve seen all of these things. If I had a dollar for every time we get told that “nothing changed”, I could retire!
What actually happened that is causing the problem can be terribly difficult to diagnose and it can be very frustrating. But whatever it was, it wasn’t caused by magic 
Hey, we don’t have that energy here 
That is actually one of our favorite lines We want it even on T-Shirts!
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I was able to resolve it and I now believe the issues are not linked. Here’s what happened:
I was sure that something must have changed, but couldn’t think of anything major.
With the search term mentioned by @dhj I found many things that didn’t solve the issue directly, but somewhere on the third page someone mentioned the app Amphetamine and all of sudden I knew what changed: To prevent the screensaver/standby while Gig Performer is running I had installed Amphetamine from the Apple App Store as mentioned in the [blog] optimize your Mac for a gig. When the app is deactivated no crashes happen.
So for my system it’s the following:
External monitor + laptop closed + Gig Performer + Amphetamine = crash on exit
Easy to solve by not using Amphetamine. 
But couldn’t there be a global option “Disable screensaver” as I know it from some other applications? 
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Totally agree, something changed, just not my doing. All im saying is I didn’t make any system changes to the Mac, but well aware these things happen behind the scenes.
I really only started this thread as a FYI, as I thought there might be something in the crash report that you could use.
I wasn’t asking for help, as the issue seems to have sorted itself out.
Very strange, two GP power users use this app. Maybe it has some weird effect with the specific version of app on a specific macOS.
I have added a note so that people are cautious when using it.
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I’d never heard of it until today, so it’s not causing my issue.
The Skylight framework is responsible for display management!