I've just cleared the paths for all the waterways. But when I got to the portal, I find there is a missing platform! (that previously, it was shown in the animation as it was complete).
Don't know if it's a bug, but I have played the Agallan part already 4 times and never had that before. You should just load the game before you started the agallan trail (if that's possible) and do it again. I think this is a possible solution.