Killing (or possibly just teleporting) Zixzax could reset an issue in D:OS Classic, but will not help in the EE.

Do you have the latest update for the game (2.0.119.430 on PC)? A handful of people ran into this issue (usually with the Hall of Darkness or Trader's Emporium) and there was a fix in one of the updates.

Please email supportdos@larian.com, with your latest save (or a Dropbox or Google drive link, etc).
Each save is a folder in the '..\Documents\Larian Studios\Divinity Original Sin Enhanced Edition\PlayerProfiles\<ProfileName>\Savegames_patch' folder in Windows and OSX, and the corresponding folder in the home directory for Linux.