I usually take a look at the code, but I don't see your code. This is from the code in your link.
// buffers for receiving and sending data
//char packetBuffer[UDP_TX_PACKET_MAX_SIZE]; //buffer to hold incoming packet,
char ReplyBuffer[21];
If you use ReplyBuffer with more than 20 characters plus the terminating zero, you're overflowing the array.
If you use packetBuffer above, this is the define in EthernetUdp.h
#define UDP_TX_PACKET_MAX_SIZE 24
It overflows at 25.