Are you working from different computer exporting to the same issue folder? In that case, go into to that folder and delta the file called ISSUE.LOCK, then start production tool again, that should do it.
Is it a specific file that is causing you problem? What happens if you remove verticals one by one of the clipboard? If you find a vertical that is causing problems, check if you are using any linking and linking to other verticals or media and make sure does works.
What happens if you remove the A-layer only feature of the vertical and re-export, does it still give you the error? If so, go to Preferences in the production tool and make sure nothing is selected under Mib versions file.
Let me know how it works out!