What solved it for me was actually an advice from ZOS:
"Looking at the tracert that you sent in we determined that it looks like the information is going through a bad route to get back to you and thus could be why you are getting the kickouts and lag spikes. To attempt to fix this please follow these steps to flush your DNS:
Click on the Start button
Type cmd in the Start Menu
Right-click on Command Prompt and select Run as Administrator
Type "ipconfig /flushdns" and hit enter
If it was successful, you will see the message "Windows IP configuration successfully flushed the DNS Resolver Cache."
Power cycle the router to clear the cache so that it doesn't store the bad route
We are finding many of you experiencing the "Unknown error" could be because of packet loss somewhere along the way from your machine to our servers. There are many things that can contribute to packets being lost, which can vary from individual to individual.
For anyone getting the error, please run the game consultant to generate a report to post here in this thread.
- Open the ESO Launcher.
- Click on the Gear icon in the upper-right hand corner.
- Click the About tab.
- Click the Game Consultant button.
- Click Run Test to create the data log.
Note: this function aggregates a MSinfo, a DxDiag report, pathping, and traceroute
- A new window should open telling you the report is saved. Click the OK button.
- The report saves itself in your My Documents folder as a text file called Report.txt.
- Post the Report.txt file in this thread.
We also suggest going to http://www.pingtest.net and running a test to see if you are losing any packets.
In addition, there are a few troubleshooting steps you may want to try that could help out with stopping the "Unknown error" that some users have had success with.
First, try removing or disabling any add-ons you are using in game. It's possible some add-ons may be contributing to getting the "unknown error."
Second, you could be experiencing signal interference if you're using a wi-fi connection. In your router settings, try changing the wi-fi broadcast signal to 1, 6, or 11 and test out if that helps or not. If possible, try a direct wired connection as well.
Third, make sure any services on your machine that could use bandwidth are turned off while playing the game. Such things as download software or communication software could contribute to packet loss as a connection gets overwhelmed while the game is running.