I have just run into this problem myself.
I can verify that in my tests, when I switched my x_redirect_url parameter to use a non-ssl host it started redirecting properly back to my site. I can also verify that Authorize.net support does not know how their own products work; I had to explain to them how the x_redirect_url works and got no usable info from them regarding any kind of validation they might be doing on the redirect URL that could cause their system to refuse to redirect.
I thought it was due to a domain name mismatch in my dev environment so I just forced my dev environment to use a non-ssl redirect. Then when I launched my new site I discovered that the problem persisted with SSL redirects on my production site, so I've just switched it back to using non-ssl redirects on my production site temporarily until I get this sorted out.
I have not yet regenerated my ssl cert because I'm not sure if it's using SHA1 or SHA2 and I don't want to regenerate it and reinstall it until I'm sure the cert is SHA1.
I'm having a hard time determining which SHA version it uses because the Thumbprint Algorithm shows SHA1, but then I see SHA256RSA for the Signature Algorithm and sha256 for the Signature Hash Algorithm. So, if anyone knows if that means I have SHA1 or SHA2, please reply to this reply.