We are faced with the same issue , SBS 2011 , updated to 9.5 U3 about three weeks ago.
Since last week, VSS network timeout error, we have reboot the server. It doesn't come right with this too , need to restart Exchange information store, after restart.
Then with the scheduled backup , it may fail once or twice or thrice and give up as total failure OR it may go through successfully in the second or third retry
Then the backup may work. Until of 9.5 U2 , things were like clock work.