[#10024] Error: "This installation package could not be opened"

Problem

When you try to install Citavi you receive the following error message: "This installation package could not be opened."

Cause

The Windows Installer service is disabled.

Solution

In the Windows Control Panel under Administrative Tools > Services, check whether the Windows Installer service is enabled. If not, click Start the service:
 
 
If that doesn't help, re-register the Installer service:
  1. Press WIN+R.
  2. Enter cmd and click OK.
  3. Enter msiexec /unregister and press Enter.
  4. Enter msiexec /regserver and press Enter.
  5. Restart your computer.
  6. Install Citavi.