There is a bunch of people having the same problem right now according to what I can read in the official forum.
I noticed that most of people complaining about that are using chars under level 100. So my best guess is that having a character over 100 will grant you permission to pass through the cauldron portal.
I am also confident about that because I have a level 110 and he can get through the portal normally.
EDIT: This requirement is no longer the same in 2020. Now you can go below level 100.