had a problem executing “ssh -X” into my new Linux box

Googled for the obvious messages, found myself in an Ubuntu forum, and I must tell you this:
Never look for help in any such forum! More incompetence than competence, you only find yourself fooled by school kids.

This solved my problem. It had to do with IPv6, which was set up for the kernel, but not for the network devices. I switched it of for the kernel then.