Transit Package Handler not working with Trados 2021

I downloaded the newest Transit Package Handler plugin 3.3.11.1 (2021) and installed it in Studio 2021 SR2 - 16.2.8.9097. When I open the PPF file from my client, there should be a 'translatable' *.sdlxliff file, but there is only a 'reference' file with the extension *.ENG. The client told me that he is able to open the same PPF file with Studio 2017.

Can somebody help me?