ENC28J60 won't receive NTP UDP requests - same sketch using Ethernet library does

[RalphBacon]

With a W5100 shield and the standard Arduino UDP NTP client sketch (https://www.arduino.cc/en/Tutorial/UdpNtpClient) , the UIPEthernet library does not process (or get, in order to process) a response.

Although it appears to send an NTP packet successfully, it never gets a reply.

The exact same sketch using the Ethernet library (and a W5100) works as you would expect. At one stage my router was reporting UDP_NULL_PORT errors (?) but I couldn't tell whether the packets were being rejected. Perhaps they were, which is why the packet was never received (and hence never processed) by the UIPEthernet library.

Whatever the cause, there seems to be a change in the way the UDP NTP packet is generated by the two libraries. I've had to revert to the standard Ethernet library in order to get the date/time when sending the occasional email, and which means I can't use my ENC28J60 module.

Any ideas?