The request failed with HTTP status 401: Unauthorized in Sharepoint logs

Below is one of the solution suggested by microsoft in this types of issue

1) Disable Loopback check on SSRS server (Method 2)

1. Click Start, click Run, type regedit, and then click OK.
2. In Registry Editor, locate and then click the following registry key:
   
   

   HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa
3. Right-click Lsa, point to New, and then click DWORD Value.
4. Type DisableLoopbackCheck, and then press ENTER.
5. Right-click DisableLoopbackCheck, and then click Modify.
6. In the Value data box, type 1, and then click OK.
7. Quit Registry Editor, and then restart your computer.

http://support.microsoft.com/default.aspx?scid=kb;en-us;896861

 

2) The next thing you want to try to check the authentication level on your IIS. This is related to permission issue. We had the simlar problem in SSRS reporting not opening a custom reporting page containing report viewer webpart in a https site

 Open IIS manager, click on the site, click authentication and make sure you have the Windows and ASP.Net impersonation enabled on the site. Check the same for the Default site if you are using it.