I was able to determine one issue related to this. The Flow was built in a Dev environment and migrated through Test to Prod. I found that the Group (sys_user_group) record that was associated with the approval step in the flow had a different sys_id in Dev compared to the other environments. The flow still showed me the correct name/display value for the group, but the sys_id in the background (obscured through Flow's menagerie of configuration records) was incorrect once the flow was in Test and Prod. I manually updated the group reference in the flow and it is working successfully.
I encountered a similar error message later on -- If an approver rejected the RITM, the flow was built to set the Stage using the flow's built-in stage manager and the first step within that stage was to set the State to Closed Incompleted. To troubleshoot this, I used the module "Debug Business Rule (details)" and the Script Tracer tab to keep an eye on all the business rules that were running.
I found that the 'task closer' business rule was causing the RITM to be marked as Closed Complete and then the Flow was cancelled (all during the same transaction). I added a condition to the 'task closer' business rule to not run on the sc_req_item table, but that might be too heavy-handed... I may add some additional conditions if I find that the BR is still needed for other situations. I also removed the Request Cancelled stage from the Flow's stage handler and added it as a field that is set by an Update Record action in the flow.
It's not code, just a condition on the business rule. Like I said in my previous post, this solution is probably too extreme and should probably be refined, especially if your workflows/flows don't handle the closure of RITMs for you or you rely on Execution Plans in your catalog.
