Will not work with payprop or xero, but if you use a webhook url without the colon it works fine. To determine this I ran the same test with integromat.
I remember this one with Xero, For some reason they only expect you to use port 443 for HTTPS and not anything else which I think is a bit silly.
The only solution to this one is to change the port you use for n8n and instead of using 5678 use port 443 instead, This can be done using a reverse proxy if you have other services already using that port.
No problem @paul2000, It is a bit of a silly issue and I am sure I raised it with Xero at my last job so they probably have multiple requests to allow non standard ports.