This "FIX" will fix it however its a problem on zenimax side on verifying the version. Re installing, or doing this fix doesn't mean it can't happen again. You have the right version, its an issue on there end. This just helps get it to verify properly.
The problem Lies with the updater and verifying with the server.
Find your zenimax folder in program files. Open it and go into elder scrolls online. You will see 3 folders. In each folder there is a respective version folder.
vo.en.version
game.version
depot.version
DO NOT DELETE.
CUT AND PASTE THEM TO A DIFFERENT FOLDER.... doesn't matter where.
Open launcher. It will probably try to re download game. It also may start the client and at same time try downloading the full game, as it is confused without the folders.
Close both the client, and the launcher.
Put the version folders BACK WHERE YOU FOUND THEM. Re open launcher. This should fix the problem. Basically shocks the launchers into working =p
You can avoid re downloading the game by doing this, however it is NOT ADVISED as ANY AND ALL messing around with these folders could cause you to have to re install the entire game, so backing up the game files is a extremely good idea.
To TESO team, would be great to have a tool made that does this kind of thing when you get error 209... re downloading the game is just a horrid hassle.
Its all because the login and authentication servers are getting confused with the version number, or at least that's what it looks like..
This is game breaking and stops people from playing... a tool.... yes a tool would be great...
UPDATE FOR MORE INFO
delete bethesda_net.launcher.version" file from the "\Zenimax Online\Launcher" folder *****
this has worked for some and for others the game has tried to fully re download. DO NOT DELETE. Move it out of the folder so you can put it back if it does not work.
If you delete the folders, as another poster mentioned.
https://help.elderscrollsonline.com/app/answers/detail/a_id/6486/
will show how to get them back
This "FIX" will fix it however its a problem on zenimax side on verifying the version. Re installing, or doing this fix doesn't mean it can't happen again. You have the right version, its an issue on there end. This just helps get it to verify properly.
IP FIG commands ect -- from another post that may help.
http://forums.elderscrollsonline.com/discussion/80209/found-the-root-cause-of-many-login-issues#latest