I was looking at my LR catalogue (LR Classic 2021) and noticed that some of the older photos don't load. The photos are DNG files from 2014/2015. Lightroom displays "The file appears to be unsupported or damaged". I can open those DNGs in Photoshop (Camera Raw and editor) as well as in Windows Photos app. Only Lightroom seems to have a problem with them.
I even restored them from a cloud backup - the same behaviour.
No I have not seen this at all. I have many Native DNG files from 2007 to 2014 from Leica M8 cameras I've owned and DNG converted from Nikon NEF files from 2006 through 2008. All of them work fine in LrC V10 catalog upgraded from a LrC V9 catalog.
I suspect the actual files that LrC is saying is Unsupported or damaged is actually damaged, corrupted, and even your online backups are too. Do you have a local, REAL, backup of those files?
Once thing you could try is Deleteing the LrC Preview folder and let LrC V10 Rebuild the preview of those files.
The files are converted from Nikon NEF files in Lightroom, at the time of the import, namely in 2014. They must be damaged somehow, as plenty of other photos from around the same time are working just fine. I do have a local backup, which is my first line of defence, but the files there are binary identical to my working copy and cloud backup (verified a handful of problematic ones).
As the DNGs work in Photoshop and virtually any other app, I can still convert them to TIFFs and reimport, but it will be quite an exercise, as we are talking about a few hundred of photos segregated to various albums. Look like I have to swallow it.