[badkarmaboy]

Thank you for your reply. I checked both profiles (master and secondary) and as you said, there was no font set. I have set it to "default" in both profiles, then restarted.
Sadly the issue persists, even when setting it to "Arial" and rebooting.

Strange, other users reported that it was the solution. Once you set the font, the labels will apear immediately. No need to restart.
Also, you should only have to set it once, after that all should be fine.

For the meantime I use XBMC.ReloadSkin() in autoexec.py:

That really should not be needed. If the above solution doesn't work for you or the setting isn't saved, something else is wrong on your setup.
Calling ReloadSkin is a last resort option.

By calling script.skinhelper.service I hoped for instant updating of the "Random [Movies | TV Shows] Backgound" (as the kid gets to see the adult movies / tv shows backgrounds after startup), but it looks like it's not working reliably. I have the feeling I need to restart the service or pass parameters to it, but I don't know how. Marcel, maybe you can help me?

Don't do that. Skin helper is a service which is loaded by Kodi in the background.
I will fix the wrong backgrounds loaded in the code for you by adding the profilename to the cache.

Great, thank you once again! I'll remove the manual Skin Helper call.