Citavi's Word Add-In cannot be run if the following option in Word's Trust Center is enabled:
Disable all Application Add-ins (may impair functionality)

Solution

  1. Open a Word document.
  2. Click File > Options.
  3. Switch to Trust Center.
  4. Click Trust Center Settings.
  5. Switch to Add-ins.
  6. Check if the option Disable all Application Add-ins (may impair functionality) is enabled. If it is, clear the checkbox.