Zabbix http test doesn't works against web server with challenge authentication enabled (all modern sharepoint site). Web Server ask for negotiate authentication, rise a 401 then fallback with ntlm authentication. Zabbix catch only first 401 and exit.

Test login with curl via command line works fine. Check the curl challenge authentication:

• About to connect() to sharepoint.domain.com port 80
• Trying 192.168.10.22... connected
• Connected to sharepoint.domain.com (192.168.10.22) port 80
• Server auth using NTLM with user 'domain\zabbixusr'
  > GET /default.aspx HTTP/1.1
  > Authorization: NTLM TlRMTVNTUAABAAAAAgIAAAAAAAAgAAAAAAAAACAAAAA=
  > User-Agent: curl/7.15.3 (x86_64-unknown-linux-gnu) libcurl/7.15.3 OpenSSL/0.9.8w
  > Host: sharepoint.domain.com
  > Accept: /
  >
  < HTTP/1.1 401 Unauthorized
  < Server: Microsoft-IIS/7.5
  < SPRequestGuid: ba4974ab-5d3c-4d7f-960f-9463784ea074
  < WWW-Authenticate: NTLM TlRMTVNTUAACAAAABwAHADgAAAAGAoECkPi1LQAnQhAAAAAAAAAAAJ4AngA/AAAABgGxHQAAAA9DUklGTkVUAgAOAEMAUgBJAEYATgBFAFQAAQAUAFMAUABJAE4AVABQAFIARAAwADEABAAWAGMAcgBpAGYAbgBlAHQALgBjAG8AbQADACwAUwBQAEkATgBUAFAAUgBEADAAMQAuAGMAcgBpAGYAbgBlAHQALgBjAG8AbQAFABYAYwByAGkAZgBuAGUAdAAuAGMAbwBtAAcACADGSKUCfybPAQAAAAA=
  < WWW-Authenticate: Negotiate
  < X-Powered-By: ASP.NET
  < MicrosoftSharePointTeamServices: 14.0.0.6123
  < X-MS-InvokeApp: 1; RequireReadOnly
  < Date: Mon, 10 Feb 2014 16:41:56 GMT
  < Content-Length: 0
• Connection #0 to host sharepoint.domain.com left intact
• Issue another request to this URL: 'http://sharepoint.domain.com/default.aspx'
• Re-using existing connection! (#0) with host sharepoint.domain.com
• Connected to sharepoint.domain.com (192.168.10.22) port 80
• Server auth using NTLM with user 'domain\zabbixusr'
  > GET /default.aspx HTTP/1.1
  > Authorization: NTLM TlRMTVNTUAADAAAAGAAYAFcAAAAYABgAbwAAAAcABwBAAAAAEAAQAEcAAAAAAAAAVwAAAAAAAACHAAAAAYIAAGNyaWZuZXR6YWJiaXh1c3JjcmlmbmV03y3hJz8rHzE31htWl6cezJ9JlW0mUjbIIAhzvteHruuyzJaKFpjlKrA309j5xHsL
  > User-Agent: curl/7.15.3 (x86_64-unknown-linux-gnu) libcurl/7.15.3 OpenSSL/0.9.8w
  > Host: sharepoint.domain.com
  > Accept: /
  >
  < HTTP/1.1 200 OK
  < Cache-Control: private, max-age=0
  < Content-Type: text/html; charset=utf-8
  < Expires: Sun, 26 Jan 2014 16:41:56 GMT
  < Last-Modified: Mon, 10 Feb 2014 16:41:56 GMT
  < Server: Microsoft-IIS/7.5
  < SPRequestGuid: eae7adfe-d0f9-4116-8d48-b30e0e8eb6d8
  < X-SharePointHealthScore: 0
  < X-AspNet-Version: 2.0.50727
  < Set-Cookie: 6e9a6995ab344b25b19e0963e0c99a2ci%3A0%23%2Ew%7Cdomain%5Czabbixusr=0; expires=Tue, 11-Feb-2014 16:41:56 GMT; path=/; HttpOnly
  < Set-Cookie: WebAnalyticsSessionId2=b0cfd857-b0c6-4fec-9a29-346c8eb24acc; path=/
  < Persistent-Auth: true
  < X-Powered-By: ASP.NET
  < MicrosoftSharePointTeamServices: 14.0.0.6123
  < X-MS-InvokeApp: 1; RequireReadOnly
  < Date: Mon, 10 Feb 2014 16:41:56 GMT
  < Content-Length: 62504