Hi Dan C,
Yes, there were two files from Affinity in action, but I can no longer trace the second one:
Attempt at a reconstruction:
I had two files with the same project with different states. I wanted to insert a graphic that was linked in one Affinity document into the other, current Affinity project with the Copy command. After I had done this, a call came up that was something like this: I should close the document. After that I could no longer open it - I don't think both documents any more - and the error "the document would be faulty" appeared.
I hope that you can analyse or isolate the problem better this way.
Have a nice day!
Klemens