When running a SmartObject or Form there is an error shown related to the App Only token. This issue is usually seen after a K2 Service Account password is changed.
"Message=10702 An error occurred in the [SHAREPOINT] Service Instance. SourceCode.Hosting.Exceptions.AuthenticationException: Error issuing app only token. ---> System.Exception: HostServer.ConfigurationValue('[K2_SIGNING_CERTIFICATE_PRIVATE]') is empty."
Re-running the K2 Setup Manager and making sure the correct password is entered should resolve the issue.