WSUS catalog synchronization depends on connectivity to the upstream server or Microsoft Update.

A possible fix is:

1) Start WSUS 3.0
2) Click Options
3) Click Update Source and Proxy Server
4) Review update source settings and make changes as needed
5) Review proxy server settings and make changes as needed
6) Click Synchronization
7) In the action pane click Synchronize Now

If the synchronisation is successful the above error may have occured due to a minor network issue.