@patcon, yes the code was blindly copied from test-kitchen where a full clean is wanted. Obviously we don't need it. Not even the cache. Feel free to submit a PR to get full credit out of this if you want. =)
Anyway thanks a lot for testing and finding it!

@matschaffer, I also experienced some issues with earlier Librarian versions, but it seems they got fixed in some point. In those cases it was anyway enough to remove the cache (./tmp/librarian).