PrBoom for Linux / SDL problem

[TheProphet]

I hope, here are some Linux users around who can help me.

I just installed PrBoom on my Linux system.
When I run /usr/games/prboom, it says:

<snip>
prboom v2.1.0 (http://prboom.sourceforge.net/)
Z_Init : Allocated 8064Kb zone memory
Xlib: connection to ":0.0" refused by server
Xlib: Invalid MIT-MAGIC-COOKIE-1 key
Xlib: connection to ":0.0" refused by server
Xlib: Invalid MIT-MAGIC-COOKIE-1 key
Could not initialize SDL [No available video device]
</snip>

Any idea what the problem could be?

or

Is there a source port for Linux that is easier to install for a *n*x newbie like me?

[TheProphet]

I figured it out myself. (In case anyone is interested.)