SerOut

The module needs 9600 baud, no parity, 8 databits and one stopbit.

I changed the baudrate again. This time I used a baudrate like 9200 and my error code changed from these weird numbers to
! c y

This is almost correct. If the "c" and "y" would be capital letters, then it would work.

So I thought of changing the baudrate to other numbers, but I cannot get rid of the error message.

The module still reads small letters instead of capital letters.

Are you using 232 or 485 or something else? It looks to me as if you may be letting lines drift. Do you have 0V connected properly and are there weak pull-ups on the data lines (weak pull-aparts for 485) to ensure that data lines are held in the correct idle state?

Merlin