In one of our SharePoint Farms we recognized, that suddenly some SharePoint Workflows shows a message “Failed to start”. We tried to manually restart them, but the error comes again. After investigation of error logs and workflows in SharePoint Designer we realized that only some of the workflows didn’t work. Those who uses conditions and special tasks belonged to the workflows not working anymore.
One of the error messages was:
“System.CodeDom.CodeBinaryOperatorExpression is not marked as authorized in the application configuration file”
So we tried to republish the workflow. But SharePoint Designer throwed an similiar message, that activities or conditions could not be found. But only at workflows which didn’t run. We could republish working workflows.
After some more research, we found a really good article, which helped in this situation:
So the reason was an security patch. After adding the entries into the web.config of SharePoint the Workflows worked again.