This normally has something to do with the keys: UrBackup - Frequently asked questions
I am not 100% sure it is the case, since you are saying they have connected to the server…
- Are you out of drive space to hold images?
- Inside the web-interface, what are the statuses of the clients that cannot backup?
- Is your Max Simultaneous Backups set too low?
- Did you disable file or image backups for your global or client specific settings?
If you need to, you can send a log report in by following this: