Sometimes that can happen if you are going to a URL that does not match the callback URL you gave to Facebook.
Does your site answer to multiple domains?
I notice that if I go to <!-- m --><a class="postlink" href="
b3dmultitech.com/">
b3dmultitech.com/ it does not redirect to
www.b3dmultitech.com/">
www.b3dmultitech.com/, but both links still bring up the site. Can you see if that's what causes the issue? If so, I would suggest having one redirect to the other using your htaccess file. Let me know if you want help doing that.
If this isn't the issue, let me know what I can do to reproduce it and I will try to help. Is it only with a certain browser? Is it when you are already logged in to Facebook?
Thanks