Unable to cast COM object of type 'System.__ComObject' to interface type 'Interop.QBXMLRP2.RequestProcessor2'

You must install the QuickBooks SDK

Problem

When you run the web connector, you get this error:

Unable to cast COM object of type 'System.__ComObject' to interface type 'Interop.QBXMLRP2.RequestProcessor2'

Description

You must install the QuickBooks SDK. Some files are missing.
Download QB SDK 14.
Download QB SDK 15.

Solution

Install the SDK

1. Go here: C:\Program Files (x86)\Intuit\IDN\QBSDK14.0\tools\installers
2. Double click QBXMLRP2Installer.exe:
3. Reboot.
4. Run the web connector again.