I was unlucky as well, after 25h I good a disconnection with one arduino, though that is the one which used to disconnect after a few hours (I am using the arduinos in connection with FHEM). Any ideas for further investigations ?

All Arduinos: Running since almost 12 hours, looking good so far . I will keep you updated. Update: 18h and no lost connection Update 2: 24h and no lost connection, looking good

@ntruchsess Thank you very much. I have uploaded the new version to the three Arduinos just now. So we just have to wait and see....

@frol I will try it as soon as I am back at home, probably tomorrow evening. At the moment I am abroad without access to the Arduino..

After 2-8 hours. After that I have to repower. Before that it doesn't respond to Ping.

I'm using an Arduino Nano and still they are loosing connection regularly

Shouldn't the ENC28J60 be connected to 3.3V. I know it is 5V resistant, but still. May be it makes a difference ?
@frol Have you been able to run some tests with ESTAT ?

I tried it with a sketch. After approx. 4 hours the arduino disappeared. No ping, no TCP communication. There has to be a different problem...