Do you have a lot of addresses in Armory?  This frequently occurs when Armory hits the 10-20 sec timeout during an operation... it happens most frequently when there is network burp, or you have 10k+ addresses and it's trying to update all of them after a new block comes in.  If it's an address problem, we'll have a fix for it soon ... the 0.91-dev branch already has updates that make Armory operate much better with 50k+ addresses, though there's still more work to do.