Based on the fact that I don’t need all the feature eGroupware is providing, I switched to CE-Version.
More details are here → https://www.egroupware.org/de/egroupware-ce-vs-epl
The steps to disable are explained here → 🇩🇪 🇬🇧 EGroupware Community Edition auf/on UCS - EGroupware on UCS - EGroupware Help Forum