Pesti,

In your program you have the following line;
HSerin 10000,Main_RFID,[Menupunkt]
This command tells PicBasic to wait ten seconds (10000 milliseconds,)for input from the hardware serial port, before it moves on. The OnInterrupt command doesn't really interrupt, it just sets a flag, and waits the ten seconds before it moves on. In your second example you have shortened this to 1000, which means that it will only wait one second instead of ten. However, if you start using Darrel Taylor's Instant Interrupts (THANKS AGAIN DARREL!) it will interrupt instantly! Also, with Darrel's tool it is pretty easy to have different interrupts for different events, so you can have one interrupt handler for Timer0 and a different one for serial input, and another one for something else.

At the end of this line;
Lcdout $fe,$C0," ",DEC2 Stunde[0],":",DEC2 Minute[0],":",DEC2 Sekunde[0]," "
You have a space at the end? I don't think that you need it, and this might be causing your Error108 problem.
I can promise you that you will not find an easier language to learn than PicBasic, but it does take time. I suggest that you try the examples on Darrel's webpage, start with the Blinky program and work your way through them all until you understand how they go together, and remember, this is supposed to be fun!
Good Luck!

Jerry