HELP OLAP Cubes

When I try to modify a OLAp Cube, suddenly appears a Error message

"Unknow Error. Restart the Application"

And this break my cube down

This is not always, just some time, for example when I proof the cube on Microsoft Excel or After I backed up the databes of analysis

What can I do?
I'll thanks your quick answer

NOTE: My cube use MOLAP aggregationsHave you applied analysis services SP3? I've never seen this error but I know that the service pack fixes quite a few bugs.

HTHsql