I got that same error. After some troubleshooting, I discovered that my datastore wasn't reponding to calls from my exchange server. I re-cycled my box that holds my datastore and everything started working again. The best way to determine if something is wrong with a particular app, go into console manager on XPA and select 'properties' of the app that's giving you problems. If your datastore isn't responding, that's where you'll find out by the message that pops up.