Signup is being prompted instead of page

We have setup MultiSite with the latest versions of (MultiDomain and DomainMapping) at our site

The site and subdomain ( seems to be working fine (sharing the same login and everything); but randomly and on certain devices guests are prompted with the signup form and they are forced to register before of being able to see the content. We don't want this behavior, any idea of how to fix this?

Best regards.