steff22, the answer you want is one post above yours.
Yann confirmed at post #4 that AMD iGPU passthrough doesn't work on XCP-ng today and that he's working on it, so your Invalid PCI ROM header signature looks like the known symptom rather than something specific to your board.
He also linked https://xcp-ng.org/forum/topic/8909 , same error string, if you want the longer history. I don't know enough about the ROM BAR side to add anything useful to what he wrote, and I'd rather not guess in front of him.
Worth following this thread, he said he'd keep at it.