The modules (Uikit3) aren’t loading and I don’t understand which extension/plugin is missing or what’s wrong with the syntax in the articles (for example: {loadmoduleid 192} which is the homepage in Dutch) that are linking to the modules.

My test site:
mig.gitelabellevie.com/portal

In order for the UIKT plugins to work, the UIKIT 3 plugin must be downloaded and activated.

The uikit3 plug-in had been installed but it doesn’t work yet

Hello, please more details!
Whats not working?
Uikit must be installed and set to active
www.joomlaplates.com/uikit-3-dokumentation/uikit-3-plugin.html

UIKit 3 plug-in is installed and activated.
I have added a UIKit 3 module to an article, but when selecting it in the front end it is a blank page

Hello, then you did something wrong.
Maybe your modul is not published or only pubished on some menu items.
But finally its a Joomla knowledge and has nothing todo with the template