I tried to reverse engineer an Android APP using MITMProxy, and the Server responds with an unauthenticated-error.

If the whole authentication would be using headers (Basic, etc...), the Proxy should forward those headers. And it does forward the API-Key-Header.

Yet, the handshake seems to fail.

Would could cause that error?

The only thing i can imagine is a client-side SSL-certificate, which gets replaced by the Proxy, but i fail to find any code setting that certificate...