Multiple users of my SharePoint libraries are having issues syncing files at the moment. We use the site for collaboration, and need to be able to upload files to a separate content management system and often have to email files to other users that do not have access to our libraries. Because of the syncing issues, we are unable to locate files through File Explorer or Outlook. We've tried unsyncing and resyncing to no avail.
Are you using OneDrive for syncing data? If yes, have you tried to remove the reference from the OneDrive web interface and then do the sync one more time? Sync and unsync may not be enough to fix the issue.
Have you tried to see what the affected users have in common (ie: same permissions in SharePoint, belong to an active directory group, etc..)?