Per caso hai qualche richiesta in GET?
Potrebbe essere che qualche richiesta scrive troppi caratteri nella query string.

Se puoi butta in POST, altrimentdi devi modificare il web.config

codice:
<configuration>
  <system.web>
     <httpRuntime maxRequestLength="x" maxQueryStringLength="y" maxUrlLength="z"/>


414 - Request-URL Too Long
The web server responds with this error when it is refusing to service the request because the Request-URL is longer than the server is willing or able to interpret. This rare condition is only likely to occur when a client has improperly converted a POST request to a GET request with long query information, when the client has descended into a URL "black hole" of redirection (e.g., a redirected URL prefix that points to a suffix of itself), or when the server is under attack by a client attempting to exploit security holes present in some servers using fixed-length buffers for reading or manipulating the Request-URL. Typically Web servers set fairly generous limits on length for genuine URLs e.g. up to 2048 or 4096 characters. If a long URL is valid and you receive 414 errors, then the Web server may need to be reconfigured to allow such URLs through.