Thank you Jahn!

Confirm your fix also works with x2goserver 4.1.0.2-0~1642~ubuntu16.04.1. I commented the  BIG-REQUESTS line in /etc/x2go/x2goagent.options. Rebooted the session and QT apps are working again.

To fix the performance issues I also changed to XFCE instead of Openbox which improved performance significantly. I think Cairo-Dock was ultimately the issue but I'm happy with XFCE so moving on...