This issue is likely due to a linked attachment not being found during the deletion process.
Find the responsible file
To find the responsible file, browse to the attachments in the workstep and open each one. If you get an attachment that can't open, that's the responsible file.
Ensure it's on Alfresco
If you're still having issues, ensure that the attachment is on your document storage server such as Alfresco. As above, ensure that you can open the files. If the affected attachment is missing or corrupted in Alfresco, it either needs to be restored, or the link to the attachment needs to be deleted from the Mosaic database.
If you need to remove it from the database, please raise a new case online and reference the title of this article. Ensure to include the details of the file that you need removing.
Error logs
If you have access to your error logs, you'll see the error show as the following:
'CmisObjectNotFoundException: Object not found
DeleteStepServiceImpl.deleteStepDocuments(DeleteStepServiceImpl.java:266)
deleteStep_aroundBody6(DeleteStepServiceImpl.java:209)'