Recently, I got the following error message when I tried to start an X-application under Linux:

Some fonts seem to be missing in your system, you should install either xfonts-100dpi or xfonts-75dpi and then restart Xorg to get xboard running.

I got this error message despite having the packages xfonts-100dpi and xfonts-75dpi installed. After scratching my head for some time I found the problem: I was running the application via ssh with X-forwarding from a Windows PC. And the Xserver was actually running on Windows, namely the Xming application.
The problem was quickly fixed by downloading the font packages from the Xming page.