HTTP errors are meant to give you information about the communication between your device and a website server. But the codes were written for developers, not average people. And even experts can be confused by obscure codes like this one.
There can be numerous causes of a 431 error code, but the code itself translates to the following error...
431 Request Header Fields Too Large
Servers will often produce this status if:
In practical terms, the first thing to try is to resolve this issue is to use incognito mode in your browser. If you are restricted from using incognito mode the alternative is to clear cookies and browser cache. Secondarily, try a different browser.
In reality there are numerous issues that can cause this and if clearing your browser cache does or using a different browser does not help, reach out to Domino Support for assistance.