I ran into this today while installing a plug-in for a customer.  A bit of research turned up articles by Ronald and George which ultimately led me to the following KB article.

The problem lies within the fact this installation has CRM on port 81, so the URL being used internally by the plug-in is incorrect.  Here is the solution, as identified in the KB article:

Additionally, create a LocalSdkHost registry key on the Microsoft Dynamics CRM 4.0 server. To do this, follow these steps:

  1. Click Start, click Run, type regedit, and then click OK.
  2. Locate the following registry subkey:

    HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\MSCRM

  3. Right-click MSCRM, click New, and then click String Value.
  4. In the Name box, type LocalSdkHost.
  5. Right-click LocalSdkHost, and then click Modify.
  6. In the Value box, type the name of the Microsoft Dynamics CRM server or the host header, and then click OK.
    Note Do not specify http:// or the port number.
  7. Locate the LocalSdkPort key at the same location. Verify that the port that is listed matches the port that is being used for the CRM Web site.
  8. If the value of the LocalSdkPort key is incorrect, right-click LocalSdkPort, and then click Modify. Type the correct port number, and then click OK.

 

The LocalSdkPort value was already in place and correct.  After the addition of the LocalSdkHost value, everything worked as expected.

 

Note: Hotfix rollup 1 was installed on the server