Error message during a migration of a Power App from one site to another site in the same tenant

I created a customized list form based on Power App. I migrated the list using PnP Provisioning and migrated the Power App using FlowPowerAppsMigrator by Denis Molodtsov. I got the following error: "An App already exists for this list. Remove the existing App before adding a new one.". 

The error message is annoying because this app is not already existing. I compared the lists in source and destination and the string transformations made by Denis FlowerAppsMigrator. All good.

It took me some time to get the root cause. The base language of the source site is German, the base language of the destination site is English. I tried to import the package from the German soruce site to a German destination site. It worked. The same behavior as with the old site templates.