@JoRwe are now having the same issue that you are. So far the exact same issue has been confirmed on 2 separate QB Desktop 2021 computers (on both computers we applied the Temp folder solution provided by  @BigRedConsulting). We were so overjoyed by the @BigRedConsulting solution that we've been willing to live with the fact that we haven't been able to close QB on them for a few days. But this will need to be addressed eventually. Anyway, 5/31 is coming soon...tick...tick...tick...tick