[Baertram]

I'm just wondering how the addon can change your game files? This must be some kind of server setting then, isn't it?
Otherwise all the files we are possible to change have been in these directories.
Weird.

Idea:
Did you try the repair function in the launcher?

Another idea (after re-installing. This is what I do):
After a fresh installation + patching copy the whole ESO's launcher and game directory to an external harddrive, or another folder/harddrive.
After each patch you would be able to recopy the last files again from this external harddrive/other directory (assuming that you synch both folders some time after each patch, as you were able to play properly).
This helped me too many times in the past and saved me many hours of reinstalling ;-)