What Frank said is exactly what I was trying to say. The thing you’re doing, with the URL frame, is what strongly recommend against doing.
Cookies are an important reason for that. Browsers nowadays tend to be quite cautious with iframes, because they are often used for tracking, so browsers, especially those that have been tuned for extra security, will have problems with cookies, which means many website functions will not work correctly.
And that’s not considering the fact that you can’t do direct links to pages, you will not be able to rank your custom domain in search engines, and all other issues associated with URL frames.
So that’s my verdict: the issues you’re having are caused by your URL frame. To fix this, either just do a regular web redirect without frames, or actually host the domain name with us so you can publish your website on your domain name: