No, unfortunately not. I changed the code according to the comments from @kersing and I was at least able to the join ack responses in TTN and on the gateway - but they are not received by the node.
I changed the firmware to the actual version(2.1.16) and now the join requests didn´t even reach the gateway. @brady_aiello Since I think something is wrong on the node - do you know any more debugging possibilities there?
I am also wondering if my setup of gateway and node is caused some of the problems, because they are located in the same room?