After the update for PTS all my addons and screenshots are gone. I don't know if it has anything to do with the new user license or something, but something messed up.
Maybe it also was a windows update at the same time.
Check if you got OneDrive enabled and move the users documents folder away from it as ESO is not looking into the OneDrive directory if you got the documents in there.
Search for the file USerSettings.txt or AddOnSettings.txt to find the folder where the addons are currently located.
They got ZOSed, check Out of Date addons box, as the API number got updated, until the addons all get updated
or update the API number in each, but at your own peril...
“Be as a tower, that, firmly set,
Shakes not its top for any blast that blows!” Dante Alighieri, The Divine Comedy
I just had the exact same problem and after about 40 minutes of tracking it down I found the answer.
After the latest Windows update "1903" I turned on "Controlled Folder Access." This was keeping ESO from accessing my Addons folder. To fix your problem open "Windows Security" go to the "Virus & Threat Protection" tab and turn off "Controlled Folder Access."
Instead you might be able to just let ESO though but I have other games and I didn't want the same problem happening to all of them so I just turned it off.