I ran into this error this morning, and found the solution on the official forums. I'm posting it here because the way it is written over there is stupid.
If you're getting error 209 that the launcher can't determine your game version. Do this:
- Delete the .version files for the game and launcher, there are 4:
- Inside "Launcher" folder, delete "Bethesda.net_Launcher.version",
- Inside "The Elder Scrolls Online" go to the "depot" folder, delete "depot.version"
- Go to the "game" folder and delete "game_player.version"
- Go to "vo_en" folder and delete "vo_en.version".
- Launch your client. Don't click the "Install" option (which is what Repair changed to when deleting those files).
- Close the client.
- Go to your recycle bin, and restore the 4 files you deleted.
This is similar to deleting your .cache files for WoW.