If there is a problem with one of your Vault, probably because the local disk image is corrupt, the "UnableTo Sync" status will appear.
- open the dashboard
- right mouse click on the FileServer that has the UnableToSync status
- select "sync now"
Note: while your Vault has the UnableToSync status, it is in ReadOnly modus. We do this to prevent you from changing and/or adding data and avoid dataloss.