Hello Tobias,
most likely you are not matched as to T/N and baud rate to your serial LCD. I am not familiar with your particular serial LCD, try some different baud rates and try them true and inverted. I would just make several serout statements with a string to identify, the one you can read is the one you use, example:
Code:
Serout PORTC.6,N9600,["9600 inverted"]
pause 500
Serout PORTC.6,T9600,["9600 True"]
pause 500
Serout PORTC.6,N2400,["2400 inverted"]
pause 500
Serout PORTC.6,T2400,["2400 True"]
pause 500
add the rest of the available choices and you have a test program to sort out the settings of your LCD.