I don't think it has to do anything with Hosting, Can you tell us the exact error message they get.
What I suspect is because you are using bravenet services thier browser is rejecting cookies from frihost.They will need to turn on "allow cookies from 3rd party" in their browser setting in order to sign in to forum.To enable it in Internet Explorer, go to Inetrnet Options >>>Privacy Tab>>Advanced Button>>Check Over ride automatic setting and then tick allow 3rd party cookies.
Some redirect services don΄t allow cookies...(dot.tk for example). so try it with another redirect url, if your problem persist, come tell us...
Well i figured it out, i cannot cloak the redirrect url, for some reason it conflicts with the logins. I dont know if this is something to do with the server or my forums.....i would really like to have all my users use one link and without cloaking once my users click anything it changes the link in thier browsers.