Backup schedule not fulfilling

Have Urbackup Server under windows running 4 clients, all windows (Vista, Win7). Incremental file backup window is 22 hours, backup window every day between 3:00 and 6:00 o’clock in the morning. Do not execute backups is not checked. Client (Vista) shows up red after 22 hours, but is not executing backup in thw window. Right Mouse click does not offer any backup task, only status, protocols, …
Server as well as client were up and running during backup window, no network issue, client can access server (is linked through same network switch) via remote access or UrBackup web access. Client shows no protocol after last successfull backup, server protocol shows no event with regard to client.
Surpringly one of the client executed after two days of inactivity, another one currently not.

Any idea what could be wrong ?