As you perhaps remeber I used your wonderfull shieldbuddy in combination with a Industial Shiel M-DUINO21. unfortunately they discontinued the production of M-DUINO 21 and made a new model M-DUINO 21+ which uses the w5500 chip.
With the original Adruino Mega 2560 inside I can use the Ethernet2 libary out of the box.
The standard Arduino Ethernet2 shield does work with the ShieldBuddy on the default SPI port but we have not tried the new MDUINO21+. The old MDUINO21 with the W5100 on the BOARD_SOFT_SPI_SS0 SPI port works OK.
I think we will need a MDUINO21+ here for testing to find out what is wrong!
That is strange as I would expect the default SPI channel to be the one on the 6 pin header on the Mega. This suggests that using pin10 with the SPI pins on pin 50/51/52 should not work. Looking at the Ethernet2 shield library source code, it seems to be using the 6 pin header.
You could also try editing the W5500.cpp in the library to change all the SPI.transfer() commands to include the pin number BOARD_SOFT_SPI_SS0:
SPI.transfer( BOARD_SOFT_SPI_SS0, 0x01 );
The W5100.cpp for the old Ethernet shield was like this.