In Servicedesk and Workflow 8.1 RU5 and 8.1 RU6, a couple of new issues have been observed:
Resolving these issues requires upgrading the Workflow Server on the affected server.
Workflow Rollups are available for download in the following KB article:
TECH249711: Workflow and Workflow Rollup Build Links
Download the Workflow Server installer executable and upgrade the existing environment.