Warning: No xauth data; using fake authentication data for X11 forwarding
For me this issue was caused by my Fedora system not having an ~/.Xauthority file. I created one by executing the following commands on my Fedora system (Client machine):
x11-forwarding
X11 forwarding request failed on channel 0
Adding the -v option to ssh when trying to log in will give a lot of debug information which might give a clue to exactly what the problem is, like for instance which in my case installing xauth on the server fixed the issue.