WIZ820io reliability problem

I use Wiz820io module connected to Atmel xmega mcu. It is a simple UDP server that sends one datagram per second. Most of the devices are working error free continuously for several months (without reset), but 10 of the 110 units broke down within 5 months. I examined the damaged Wiz820io modules. In two cases the SPI communication is working correctly, but the network connection is not possible. 8 other modules are dead; most of them show an internal short circuit. All the other components on the motherboard (eg. MCU) are undamaged.

W5200 data codes: 1427, 1434
Wiz820io data codes (on sticker): 1450, 1518
Power source: DC/DC converter Traco Power TSR 0.5-2433
Ethernet connection cables are long, approximately 80 m.

What can cause malfunctions?
Are there any design recommendations for improving reliability?
Is WIZ550io more reliable than Wiz820io?