It's worth checking out this issue we ran into (post link can be found below). It's a slightly different error message, but also related to WFStubBuildingMachine. Just passing it along in case it comes in handy. Engineering is still working on a permanent fix for us but the fix noted in the post has kept us running in the mean time.
Stub workflow WFStubBuildingMachine fails after upgrading to 6.1