First of all let me reiterate that I got the warning dialog on every single project I opened, including new ones created from a more or less empty template (control room only stuff preset). It was not just some projects, and it always named the same “missing” file.
Also I forgot to mention that I could at no time find the Remove Missing Files menu option other than disabled. Had I had the chance, I would have tried that to see if it corrected the problem. But the command was always disabled.
By the way - is this the best place to post something like this? I’m not very experienced with the Steinberg forums. Would it have been better to post in Issues? Thanks for any guidance.