As I was trying to import a work item definition into a team project (using the Process Template Editor from gotdotnet) I received the following error message:
TF26204: The account you entered is not recognized. Contact your Team Foundation Server administrator to add your account..
But I had not done any security changes (adding account or groups or permissions). What was it taking about?
Then I understood: I had added a field that contained backslashes as values and TFS interpreted it as user account! My field contained a list of suggested values that looked like this: "Category 1\Subcategory 1". Changing them to this "Category 1 | Subcategory 1" solved the problem!