Additional information: There is no API_Secret in the ENV file in the standard app package. Depending on your thousands of clients, it takes some time to migrate. For me, it was done in less than 5 minutes.
The only moment of confusion was: when installing IN v. 5. you have to fill in the name of the company in a modal frame. After the migration it took me a few seconds to realise that there is a multi-company option in IN v. 5 and I had to switch to the "other" (migrated) company (which has the same company name).
It looks like everything has migrated well.