Back when the last DLC for Oblivion dropped (the Fighters Stronghold), when I downloaded this DLC I discovered that the Mystic Emporium, would remained locked even during opening hours and required me to break in. Thankfully I was already high leveled with the skeleton key, and a dizzing amount of stealth spells and tools to pick the lock without being seen. However I digress it can still be a hassle given that the one mage in white oftens is standing outside the place wanting to get in as I do. Thusly I'm forced to wait and pray I wait long enough he's gone but the shop is still open, and plan out the hours I need to hit the store. While only a minor nuisance to my high level character, starting with a fresh character, well not so minor now since you do need to get into the place for items for certain quests if my memory serves me well.

Even years later and maybe one maintenance update, it's still occuring. Any clues to if it is fixable?

The Unofficial patches fix this.

http://www.uesp.net/wiki/Oblivion:Mystic_Emporium#Bugs