Thank you KateG , It took me a while to get in contact with LinkedIn event support, they are definitely not as fast as you.
Sadly LinkedIn Events did not help me solve this issue. They were simply pointing out all the steps i already did before.
As a result i've tried a different webinar host. I'm sorry to say, their RTMP connection allowed me to stream the webinar directly on LinkedIn in the first try. I can not make any other conclusion than that GoTo can't connect to LinkedIn Live Events using RTMP.
I'll drop my email and the response i got from LinkedIn in a message to you.