Gotcha. Can you tell me:
1) What is the URL you've put in the "Inviter Thank you Link"?
2) What is the URL that is showing the 403 forbidden?
3) Are you using any 3rd party SEF extensions?
The links for #1 and #2 should be the same, otherwise, it means there's something trying to redirect the user along the way. Also, #1 should be a full URL (
site.com/link-to-go-to), not a relative URL (/link-to-go-to). Knowing the answers to the above should help figure things out, as what you're experiencing is not the right behavior.
Thanks,
Alex