I create a number of task work items in TFS 2010 using a list in Excel. This all appeared to work OK, and I could run a work item query in Team Explorer and see all my new work items. However then I tried to open the work item query in Project I got a TF80070 error. It loaded the work items up a point in the list then failed and showed error dialog.
I altered the query to remove the work item that it had failed on and it loaded without issue. After looking at the offending work item in more detail I saw the Title field had a carriage return in it. This did not bother Excel or Team Explorer, but Project obviously hated it. Once I edited the title (in Team Explorer) to remove the carriage return everything was fine in Project.
So if you see a TF80070 error, it might be a Project/Team Explorer patching issue as forums suggest, but also check for invalid characters in query fields