Error 431 Request Header Fields Too Large

A 431 error indicates that the client is sending a request to the server with headers (usually cookies) that are too large. As a cookie is included with every request (including every image upload), large cookies make the whole page load slower.

In order to see what request the client is sending to the server you need to:

1.  Open Developer Tools. To do this, press the key  F12 (for MacOS Alt+Cmd+i).
2. Reload the page (press  F5)
3. In Developer Tools open Network tab.
4. On top of the problematic query, right-click and choose  Copy → Copy as cURL (cmd). (In Edge browser Copy request headers).
   
5. Paste the copied query into a text editor (such as Word, TextEdit or Notepad).
6. If you now see a cookie that is too long, you can delete it by selecting the Application tab. In  Storage → Cookiesclick on the small triangle.
7. A list of addresses opens. Click on the address.
8. A list of all cookies will now appear. Select the long cookie with the problem. Press the Delete key to delete the cookie.