Onedrive readonly (green locks)

Hi, I have tried all these suggestions and nothing works.  The site is a subsite which has been built from a site template.  Content types for the document library are from the parent site.  Includes folder content types.  Per-location views are enabled and default column values per folder.  I have stripped the library right back and the problem persists.  This is a huge problem as there are hundreds of project sites based on this template.  Mandatory metadata has been turned off, check out required off, content approval off... There is a retention hold policy on the site, yet other libraries on the same site can be sucessfully synced, so the issue is definitely with the library.