Symptoms
Error Logs show the following error many times:
","Error","General","1","GeneralErrorMessage","SharePointProvider.GetGroupContainers","1 An unexpected failure was encountered when contacting SharePoint to get information from the group provider : K2 Server does not have the necessary permissions on the SharePoint server or cannot authenticate with the server.
Diagnoses
Two site collections have very similar names. The first one, the K2 service account had admin rights to the site collection but the second one did not.
Resolution
Providing the K2 Service account admin rights to the second Site Collection fixed the issue, the error no longer occurs.