I rebooted the Flint and I also reinstalled PiVPN on the Raspberry. This time I used port 6000 for Wireguard and added the UDP only rule on the Flint.
It’s all working now, but I do not know why.
Funny I also have ssh port forwarding on the flint, and when I check an internet based port open page, it reports open. But 6000 reports as closed… yet wireguard still works…I must admit that I don’t understand that.