There seems to be a conflict with the dev and prod certificates. Finally, I was able to submit the following steps:
(1) go into the keychain and delete both certificates (dev and distribution)
(2) go to the Apple website and download the distribution certificate
(3) install only the distribution certificate!
(4) export p12 from private key
(5) go to the distribution profile of the Apple distribution and regenerate / download.
From there it worked fine, don't ask why ...
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