You are working in an organization in which security settings are managed centrally. Your system administrator disabled the ability to run any add-in through group policy.
If you can only work with the Citavi Word Add-In when you start Word with administrator rights, it's very likely that your organization has disabled this setting. In the Word options in the Trust Center, you will also be able to see that the Disable all Application Add-ins (may impair functionality) option is enabled and appears in gray.
Ask your IT department to undo the following setting: User Configuration > Policies > Administrative templates > Microsoft Word 2016 > Word Options > Security > Trust Center > Disable all application add-ins.