After I added/deleted fields for a WIT.
Afterwards querying by new fields results in error "TF26000: Could not connect to the Team Foundation Server. Check that you have a network connection and that your Team Foundation server is available." (Both from TSWA un VS)
Removing the new fields from the query results in fine working query.
Similar problems are discussed in MS forums
I'm using TFS2008 with VS2008. And yes, I cleared cache.