What Display Manager are you using? I was having the same problem w/ Fluxbox and GDM. It would give me that error. There was no "fluxbox" entry on GDM, so I just put it under another section. I think it was WindowMaker. I labeled the file in my /xinit folder, xinitrc.wmaker (because the error logs said that was the needed file, check those
) and changed the permissions. Then when I tried to load the WindowMaker entry, Fluxbox would load up fine.
Hope that helps.
I put this in my xinitrc file:
Im using Fluxbox .90