Just searched the whole weekend for a bug in my code and finally found out that the bug is placed in /hardware/lm4f/libraries/Ethernet/EthernetUdp.cpp.
Packets which contain 0xFF bytes will be truncated on that position when calling Udp.read(...) To fix this you have to change the following line inside of the function:
read(unsigned char* buffer, size_t len)
from:
int8_t b;
to this:
int b;
I hope this will help anyone.