When i run a deployment from TFS, I get "Microsoft Team Foundation Server
i got the same error with our own API call, for me the problem was that the service account I'm using to trigger the deployment didn't have sufficient permissions, so check those and see if it helps.
The only confusing thing is in your question, that you say it did work before, so I'm not sure if this is a permission issue
Did get a chance to review Michael's response? Please let us know if his suggestion helped.
If not, can you please provide the additional information requested?