How can I migrate a project correctly from 2021.2 to 2022.2?

Hello,
I have created a project in Bonita 2021.2 (7.13) which works. It consists of one main process and several sub-processes. After exporting as a bos file and importing in a newer version 2022.2 (7.15), I get error messages.

Error while rersolving form mapping taskinstance/"PROCESSNAME". The target bonita form with ID null does not exist.

I have set the use of an instantiation form to "No Form", as I want to avoid the green button which otherwise appears when the process is started. However, activating the instantiation form does not help to eliminate the error. The validation status of the process is OK. I only get the information that I am not using an overview page.

Does anyone have any idea what the problem could be?

Many greetings

Bodo