VisualSVN revision file problem, possibly corrupt NTFS permissions

Question

Our main repository is about 10 years old, with 20k+ of numbered revision files residing within it.

Out of all those files, 5 have gone into what I can only call a strange state on the file system, which is preventing a full/clean checkout of the repository.

These 5 files won't copy, delete or even open to edit, citing "File Access Denied" in Windows (server 2016). I have ownership of the files and full permissions. I can change security on them, but no combination will let me do anything with them. Pick any other file in the same folder and it is fine.

These files are backed up in two places - cloud & backup hd. The cloud file will attempt to download in Chrome but I instantlly get "Failed - insufficent permissions" seemingly at the point it tries to write it to the filesystem. Same with Firefox. The backup through Windows backup won't restore them either. I can restore any other revision files from these backups, just not these 5. If corrupt, it would surprise me how they have been obtained by the backups in this form when I can't even open them.

So it is like some kind of corrupt security is travelling with the file. Does anyone know how to get these files back or is familiar with the problem? Or know anything else I could try? (I'm not keen on reconstructing the repository, especially if it could happen again).