@BoA Well, the documentation seems to say
Play beep sound, use to notify events like the end of printing. If an LCD device is attached to RepRapFirmware, a sound is played via the add-on touch screen control panel. Else the web interface will play a beep sound.
You are right, it seems to be slightly more complicated than that. Even though I cannot upload my gcode file here - upload always fails with an error (?) - I was able to reduce the problem to exactly one line:
M84 XYE; disable motors
A gcode file that just contains this line will produce the error. Adding a whitespace in front of the semicolon seems to fix this problem, btw.
Just checked - that even produces the same error when pasted into the GCode command box at the top of DWC.