HTTP 400 Bad Request (Request Header Too Long) – Error in SharePoint
One of the End-User started seeing this error in SharePoint 2007 – InfoPath form submission.
Bad Request (Request Header Too Long)
1. Try deleting the browser cookies at the Client browser first. See whether it helps.
2. Check the following registry keys, MaxRequestBytes and MaxFieldLength. You can save the below registry entries to .Reg file and merge it simply!
Windows Registry Editor Version 5.00
Restart the server for the changes to take effect, and as always take backup before making any registry change.