Note that ‘buffer overlow flag’ refers to the buffer on the ENC28J60 module, it has nothing to do with the microcontroller’s memory. I doubt there is any memory corruption issue on the microcontroller, otherwise it would have behaved strangely on WiFi mode as well, or would show up even if I isolate OS from the primary network. I suspect it has something to do with UIPEthernet library not handling certain conditions correctly, like not clearing register bits or handling certain conditions that arise when there are too many broadcasts messages and so on.
What you mentioned about “look at the input buffer to see if it’s declared; check if buffer has enough room’ — yes, of course, these are the basic steps anyone has to do when writing a C++ program. As I said, ‘buffer overflow’ is NOT referring to microcontroller’s buffer, it refers to ENC28J60’s receiving buffer (hence the receiving error flag is always raised together with the buffer overflow flag). This is a hardware buffer, not allocated by the program, but exists at fixed size on the Ethernet chip. It’s not something that we can declare the size for.