I am also having an unreadable file format issue, and may be able to add something to the post from Graham1.
I have been running a Lightroom and a Capture one catalogue in parallel for several years, but abandoned the C1 catalogues recently.
I have already had some issues when changing metadata in Lightroom, I raised this here before, and a few days ago I created a new C1 catalogue (with read and write XMP enabled).
I changed my mind and deleted the C1 catalogue and then ran a full sync in PMPlus. Of my 26,000 files, 24,000 are now considered unreadable in the catalogue. You can still read them in the PM browser if you navigate to the original folder from inside PM and open them into a contact sheet view.
I'm not entirely sure what C1 has done to the XMP file, I wouldn't have thought it would have done anything but all the XMP files are now dated the 16th March 2021. And there is obviously some mismatch in the XMP data with the PMPlus catalogue that is causing PMPlus to think the raws are unreadable.
I have attached an example XMP from a raw file the catalogue now thinks is an unreadable file format.
I meant to add, that I'm on a Mac, and the number of metadata changes reported by the Full sync dialog matches the number of Unreadable files.
Cheers,
Graham