outbrain on wordpress mu

We run a Wordpress Mu installation, but when users install the plugin and type the correct url into the "claim ownership" page, it still shows " We did not find the key in the original blog URL you gave us. Make sure we have the right URL by reviewing the "Manage Blogs" section in your outbrain account. If the URL is wrong, please delete the entry and start over by going to "Claim your blog"." We have checked and double checked the correct URL but it still shows the error. Any help?