My MAG 524 has been working without issues for eight months. This morning it stopped connecting and shows Incorrect portal URL. I have not changed any settings. The URL I have is the same one that always worked.
Log into your provider's website and look for a notification or updated portal settings. Most providers email customers before migrations but sometimes it is only in the user area.
Also worth trying: power the MAG off at the wall for 2 minutes, not just standby. Sometimes a full power cycle clears cached DNS entries that point to the old server.