Smart STB is stuck on Loading Bar and never finishes?.
Portal has technical difficulties - in most cases Portals get stuck because of bad connection to the Database, unsynchronised servers, out of disk space on the server, etc. MAG devices can cache more data and may login to such portal but it's worth checking actual Portal condition and server load, etc.
Limitation of Tizen Platform with Portals that enforce strict checks - Provider must adjust security settings related to device login.
Other type of limitation from Portal it'sself - if the Portal's Display Template is changed from the Default, it may cause such behavior on all non-MAG based devices. Switching back to default template will fix it.
CDN service is not configured properly - some Portals that use CDN to distribute the load may have inproper configs causing them to get stuck while loading.