This error can be caused by several issues, however the most common is typically related to connectivity.

1. Check if there are any Service Alerts that may be disrupting access to the game. You can check this on the Service Alerts page and on the forums. 
2. Occasionally an addon may be blocking or causing an access issue to your characters. You do not need to delete your addons, but you should see if they may be causing it. To test, close out of the game entirely and then move this folder : C:\Users\user.name\Documents\Elder Scrolls Online to your desktop and then launch the game. You can move this folder back once your testing is complete. 
3. If there is a heavy volume of players trying to log in, such as during a launch, we advise exiting the game, waiting 15 minutes and try logging in again.
4. This can also be caused by a disruption or outage with your ISP. You can check the website  https://downdetector.com/  to see if there are any local outtages or ISP related issues. 
   • If it's related to your ISP, we advise contacting your ISP
5. You may also want to power cycle your home network and flush your DNS.