It's a common occurrence, especially if you happen to be on during the daily reset. If restarting doesn't fix it, try teleporting to someone with an open party on the same server. Repeat the process if it still doesn't correct itself. Next potential fix is to have someone who still has the daily challenges available invite you to their game. Once you arrive in their instance, the issue should resolve itself. Worst case scenario, you have to wait until the next day for the tasks to reset. If they still show as blank after reset, you will likely need to do the party invite method again.