Newbie Certificate Issue - SSL Handshake Error

Hello Comunity,

I'm a newbie with OPFT and certificate stuff.
I shall establish a connection with Bosch/Rexroth Company.
I installed mendelson OFTP2 and created a SSL/TLS Server certificate at
Than I installed my certificate and the Bosch certificate in the oftp2 software.
If I send files to Bosch it works fine, but if Bosch sends something to me, I always get the SSL Hanshake error.
After reading a lot about certificates and OFTP, I guess I do not understand, which of my certificate or key files I really need to install in the Oftp2 Software.
From startssl I did get a .zip file with different certificates in it:
IIS Server, ApacheServer, NginxServer and OtherServer.
I used the OtherServer Certificates and installed the
Than, at startssl I created a .p12 file (thats my private key isn't it?) and installed it in OFTP2.

Even If I this is a stupid new topic,
I would really appreciate any help, because I don't know what else to try.

Thanks in advance and Kind Regards,