Doing a conversion of a 2018 PSE catalog to a 2021 PSE catalog, the process shows it complete, then a thumb optimization and then a failure message. Conversion log shows: "conversion log: Failed since Original Catalog was open. Close older version of Elements Organizer. conversion log: Conversion Failed"
Older version (2018) was not open. Task manager shows PSE 2021 as the only app running.