Hello again Tomo,
I seem to have tracked down your issue. The only real difference between S7-SK BSP and your custom BSP is the BSP_BOARD_S7G2_SK define. ether_phy.c driver has a conditional define that's not executed when your board is SK-S7, but is probably executed when your BSP is custom because BSP_BOARD_S7G2_SK is not defined. You'll need to open ether_phy.c file and remove the bit that's grayed out on my screenshot (lines 104 to 115). Since this file is re-generated on build, I recommend you save the changes made to that file and alter its properties to read-only, to ensure it's not overwritten.
Here's the screenshot:
Regards