We are on 6.7 and see the same error.
I copied and edited two workflows, removing roles from them and adding new ones. I’ve checked that there are no references in transitions to any roles no longer in the workflow.
I tried adding the roles back and checking again, but still saw no use of the roles in transitions. I’ve specified roles for every transition instead of using “all” in the hopes of avoiding any lingering associations. Nothing has corrected the error.
Any ideas or help much appreciated.