I had a Teamcity error because I use VCS that clones with SSH and I use the pull request feature. This is known not supported feature that raised to me error:

Failure to obtain pull request provider;
Exception: jetbrains.buildServer.pullRequests.PullRequestRetrievalException: Using SSH key authentication method to extract pull request information is impossible. Please provide an access token

My problem is that after I switched to HTTP still this error occurs (although pull request does work now).