Ok, after hours of trying out different things I found the flaw in my code.
Had to remove the single quote in my rtmp-url:
from
self.rtmpsink = gst.parse_launch("rtmpsink location='rtmp://x.x.x.x/live/myStream'")
to
self.rtmpsink = gst.parse_launch("rtmpsink location=rtmp://x.x.x.x/live/myStream")
Sometimes the simplest things are the ones that cost you the most time...