It is an error in the HTTP protocol. That is the way your web browser talks with the web server.
Quote:
431 Request Header Fields Too Large (RFC 6585)
The server is unwilling to process the request because either an individual header field, or all the header fields collectively, are too large
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This is quite basic. The server blames the browser, but as you only clicked on a link put on the page by the server in the first place, I am not sure it isn't a web site problem.
When this was the web site of a smaller organisation one could contact them to try solving the problem in coordination. But I am afraid that this is not something one would want to try with such a large and autistic company.
Your description on how to do this is not precise enough (with link) for me to try to reproduce it from my system.