I usually start by reviewing the work object history of the items to see if I can identify a pattern that allows me to reproduce the scenario. If you know the point in the flow where it happens, you can review the rules executing around that time and look for places that could possibly cause stale or incorrect data to be written. For example, check if there any Obj-Save with Write Now or Commits in pre- or post-processing activities as these could cause an issue if a rollback needed to occur. You can also check to see if there are any page copy steps that may be copying stale data.

This video may also be helpful.

https://community.pega.com/video-library/support-guide-how-troubleshoot-flow-removed-errors

This usually happen when some one has updated the flow in some other Ruleset on which operator doesnt have the access, therefore Pega RuleResolution unable to find it, or it may have many reasons for this like rule get withdrawn or rule has been updated with different name or some where wrong reference got updated.

For this you have to trace the flow, check access group, check references