Just to make sure, did you follow the steps exactly for upgrading? https://www.advancedcustomfields.com/resources/upgrading-v4-v5/
Especially the part about activating 5 before deactivating 4 and any add ons? you also should not do anything between these steps, like try to edit a field group.
If you follow the upgrade steps it should work. If it does not work then you should submit a new support ticket https://support.advancedcustomfields.com/new-ticket/
