failed: error reading the headers

I have an error that occurs for some people (sometimes).

Errormessage in the client (IE)
Bad Request
Your browser sent a request that this server could not understand.
Size of a request header field exceeds server limit.
Authorization: Negotiate YIIvagYGw………..

Server log (/usr/local/apache2/logs/error_log)

[Thu Oct 11 09:24:29 2007] [error] [client 10.68.4.46] request failed: error reading the headers

Any ideas?
/Lars