O. k., you are right … sorry!
I posted several threads (Which version of openssl is necessary?, Where are the certificates of the Linux client saved?, Linux Client 3.5.1: TLS initialisation failed) for the same problem but with different questions because I do not think that it is useful to put all the different ideas in the same thread.
edit:
I am not familiar with encryption things and how to handle it. I use the mentioned desktop client on a debian linux system. Which information about my systems are necessary?