I already suspected my hideous RMII routing on a dual-layer PCB was the issue. But that lives on another day :) In my defense: This is my first real "Highspeed" PCB featuring mixed signals and 16bit DAC output (I'm still struggling with Charge-Pump switching harmonics after fat LC's and zener diode, but that will be noted in the "Future work" section of my bachelor thesis.) Oh, and the board footprint was also set in stone.