I wasn't sure if I was 'seeing things' before just now. But it's true. :(
I prepare a service.
I save it.
I exit OpenLP.
I start OpenLP.
I can't open the service I just saved.
I have saved the same service in three different locations. The same error message(with different file locations, obviously) appears each time.

I think, no, I'm sure, that this has occurred only since OpenLP 3.0. I can't open the service files in versions 2.9.4 or 2.9.5. I haven't tried anything under 3.0.1, but having read the release notes, I can't see that anything in it will fix this error.
EDIT**
I did try installing 3.0.1 on Sunday afternoon on my Apple silicon MacBook Pro. I'm still unable to read the services which I created under 3.0.
BUT, and it is a huge but.
I created a tiny service of one song and one of my custom images, saved the service and I can open it again. I saved that service to my local drive (not a cloud drive as previously) so I don't yet know if that is a contributing factor. But I have been saving (almost) everything in the cloud for the past few years with no problems in OpenLP. It would seem very strange to me for that to suddenly become a problem. Stranger things have happened. However. the good news is that I can save service files to cloud drives and open them from cloud drives.
But VLC is still missing.