Especially note the part about changing the 600 to 500 if necessary.
Alternatively check how the mythfrontend is started when you use the orbiter (use ps axf | grep myth), and then start it yourself using the same command. That way you get the output from mythfrontend to your terminal and you might be able to determine what is going on.
(You might have to add the -display <display> option to get the frontend to open on the right display though.)
In my case the command used to start mythfrontend included "-w mythfrontend", but the -w option doesn't take any arguments so the program interpreted the mythfrontend part after the -w as a plugin that had to be loaded. This plugin does not exist, hence the weird behavior.
After that I looked into the trac tickets, found that this issue had been solved a long time ago, and that lead me to the conclusion that updates weren't installed properly.