When I edit an existing Build Definition, or try to create a new one, I'm consistently greeted by an error message:
(X) Team Foundation Error:
Type
Microsoft.TeamFoundation.ClientBasicAuthCredential
in assemblyMicrosoft.TeamFoundation.Client, Version=12.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a
is not marked as serializable.
And it is not possible to edit any of the build parameters on the Process tab as a result.
I have tried uninstalling the latest Visual Studio update, repairing Visual Studio 2013 Ultimate and then reinstalling Visual Studio again to no avail.
It turns out that clearing the credentials to your TFS server from both the Windows and the Generic section of the Windows Credentials Manager and then restarting Visual Studio will resolve the issue.
I've raised the bug with Microsoft. Thank you Brian Minisi for the tip that lead to the solution.