Redirect causing 403 error

Hi. My site has some forms that talk to the client’s email marketing software. The email sign up form, for instance, posts to an external URL, where the sign up details are stored — and then there is a redirection from there back to our site, to a thank you page.

I have done similar things with other email service providers, but in this instance, the redirection triggers a 403 error. If I refresh the 403 page, the page loads. Why won’t it load the first time?

Thanks.