I did not find a solution, so I write down my problem here. Any addon I install comes with a negative result. I mean: Console seems ok, no error message, but the addon I installed is not in the addon list and also the tab of the add-on (e.g.: Manuel Bastioni Lab - tab) not visible. I checked the directory: C:\Users<my user name>\AppData\Roaming\Blender Foundation\Blender\2.79\scripts\addons\MB-Lab-1.6.5b and the plugin is there - so the installation seems ok.
The tab is missing. I restarted Blender, but nothing has changed.
Please advise how to fix this.