There is a Bug in TFS 2010 that shows a Red X On the Work Items tab in Team Explorer.
The Work Items become unavailable and when trying to get thru the Web Access you will get an error.
Microsoft will probably release a patch to solve it soon but until then…
It seems that the problem is being created from the queries section, when configuring Work Items we change or delete fields.
When you load your Team Explorer the project refreshes, the Work Item tab refreshes the queries too and when it finds changes in fields it just stops and we get the Red X.
How to solve the problem?
Step 1: Log in with Administrator User
Step 2: Load all the problematic projects to the Team Explorer
Step 3: Create a new Team Project (That you can delete later)
When the new Team Project has the focus it refreshes and neutralize the error on one of the problematic Team Projects.
Step 4: Delete all the queries
Step 5: Refresh again and delete queries until you release all the problematic projects