Hi MarceTek

Please check the horizon guide shared in previous post and other than that looking at the symptoms, it seems like your base image contains the licensing token somewhere. Please note once you install office365 on base image, you should never open Or try to activate it on base image. If by any chance you open it on base image, it may try to contact MS server and store licensing token locally which is not recommended in non-persistent environment.

Login to base image and go to the following folder:

%localappdata%\Microsoft\Office\16.0\Licensing

If you see some text files in the folder, it means your base image contains licensing token which might be the cause of the problem. In this case, you need to either reinstall O365 (making sure you never try open it after installation on base image) Or clean all local licensing tokens before creating VDIs out of it (Follow Reset Microsoft 365 Apps for enterprise activation state - Office | Microsoft Docs on base image).