Well, I had the same problem... Couldnt select SQL 2005 server backup option... I installed hotfix 6 (as linked above) on our media server... uninstalled the be agent on the SQL 2005 server and reinstalled it by pushing it out from the console... then tried to add the SQL 2005 server... guess what?
It worked... :)
This thread helped me anyway... thanx very much... :)
Regards
Conny