Having backup issues with Small Business Server 2011? I was and was getting the exact error message in the title! After some digging around it appears the error was down to Sharepoint 2010 SP1!! If left not only do you not have an up-to-date backup but you run the risk of filling up your hard drive with Exchange transaction logs!! Rather than go through the steps needed to cure this individually; here is a link to the official SBS blog that details the fix. It's actually not that long winded!!
Anyway, good luck!!
http://blogs.technet.com/b/sbs/archive/2011/07/06/potential-issues-after-installing-sharepoint-foundation-2010-sp1.aspx
