I have a RFID project where I connect wires from a RFID reader to a breadboard. Everything worked untill I made longer wires. When I connect the GND wire to my breadboard with a small wire It works. When I connect my GND wire with a long wire (50 cm, made it myself) it does not work. I tested all the wires and if I use my own made wires for other pins they work exept with the GND pin.
Can someone help me with that? Is there a max length for the GND wire?
I hope that someone can help me 
Is there a max length for the GND wire?
No but their are maximum lengths for signals. A signal will degrade the longer it is made to travel. It sounds like that is what is happening.
However you give very little details about this reader so I can't say much more.
In general trying to send fast logic signals over about 1 foot is not going to be reliable, as phenomena like
crosstalk and signal reflection become worse. And it the cabling isn't constant impedance even worse things
happen.
Please give full details of all the hardware if you want a useful answer, and I suggest also a photo of the cabling.