gtk applications are crashing in kde

For every GTK-Application I try to start a BadName error is triggered:

philipp@philipp-desktop:~$ firefox
FoxyProxy settingsDir = /home/philipp/.mozilla/firefox/33ynf63p.default
The program 'firefox' received an X Window System error.
This probably reflects a bug in the program.
The error was 'BadName (named color or font does not exist)'.
  (Details: serial 806 error_code 15 request_code 45 minor_code 0)
  (Note to programmers: normally, X errors are reported asynchronously;
   that is, you will receive the error a while after causing it.
   To debug your program, run it with the --sync command line
   option to change this behavior. You can then get a meaningful
   backtrace from your debugger if you break on the gdk_x_error() function.)
philipp@philipp-desktop:~$ gimp
The program 'gimp' received an X Window System error.
This probably reflects a bug in the program.
The error was 'BadName (named color or font does not exist)'.
  (Details: serial 1699 error_code 15 request_code 45 minor_code 0)
  (Note to programmers: normally, X errors are reported asynchronously;
   that is, you will receive the error a while after causing it.
   To debug your program, run it with the --sync command line
   option to change this behavior. You can then get a meaningful
   backtrace from your debugger if you break on the gdk_x_error() function.)

(script-fu:16539): LibGimpBase-WARNING **: script-fu: gimp_wire_read(): error
philipp@philipp-desktop:~$

It doesn't change if I switch the GTK-Style to QTCurve in System Settings