They JUST reworked how the instance system works.

This maintenance will introduce a new structure to the instance system that automatically adjusts the number of available instances according to the level of congestion. Until now, depending on the time of day, the number of players would become concentrated in certain content, maxing out the number of available instances, resulting in extended waiting times for players despite having a full party. With the implementation of this new system, more players than ever before will be able to participate simultaneously in the same content.

Because the implementation of this new structure will require a modification to the very core of the instance system, much time will be needed for checks and verifications before going live, which is the reason for the extended length of this maintenance.
They messed with the very core of the instance system trying to streamline how the entire system works, of course there are going to be kinks in the system for a while after they do something like that.

It's getting fairly stupid now how every time there is an update to the instance system and a period of working the kinks out people immediately jump on how it's obviously because the game is doing poorly and they're cutting corners by trying to use crap servers they refuse to update.