Certain SharePoint 2013 features as well as K2 for SharePoint need to have User Profile Application (UPA) working and its database populated with correct data.
Sometimes it is difficult to confirm whether or not UPA is correctly configured as SharePoint UI does not show you all the properties for the users. Moreover, even if UPA is not configured properly users still can login to SharePoint and successfully get OAuth tokern, and this fact complicates troubleshooting.
As a quick way to confirm that UPA is populated correctly for a particular user you may ask him to login to SharePoint and navigate to the following page:
It will return all UPA propeerties for the user. For OAuth tokens to work correctly following properties should be popluated: SPS-ClaimID, SPS-ClaimProviderID, SPS-ClaimProviderType, and SPS-UserPrincipalName.