Issue
When publishing Nintex Workflow, the following error may occur:
Error "Server was unable to process request. ---> The file "filename.xsn" is not checked out."
Error "Server was unable to process request. ---> The file "filename.xsn" is not checked out."
Resolution
Find the InfoPath Form that has an issue and delete it from the Workflow.
Additional Information
The issue is occurs because one of the InfoPath form (.xsn) is corrupted.