Shame, it would have been much more useful if the server had said which header it had an issue with. The longest header you have direct influence over is the Cookies header; clearing your your cookies may reduce it (but the server sets cookies, so if it's huge, it's the server's fault). Other headers can be inserted by browser extensions, proxies, etc and you may or may not have control over these
Press F12, go on the NETWORK tab, then refresh your diynot page. Click on the name column of the request that failed, to see an inspector window set in the right hand side, and choose the tab that lets you see the headers. Post a
redacted screenshot of that; do not post your cookie in clear text, draw a blank box/blur/scribble/etc over it - want to see the length of the header, not the content
For example, here's a shot of the request headers panel for this page, from my Edge:
Your headers should be broadly in line with these, size wise
