Thank you very much for the feedback from all of you.

The Byte[74] I changed to [80] already but the biggest problem was the following:

In the main routine I call the SUBROUTINE Check_RS232

Code:
GOSUB Check_RS232
In the subroutine when the 10ms counter ends and there is no data I don't do a Return but a hard goto Main. So I get unfinished GOSUBS

Code:
SERIN RS232_In,N9600,10,Main,DigiData[0]
After changing the code to check for serial data in the main routine and don't use the GOSUB the problem was solved.