Kodi 17.4 instead of 17.6 ??

[Nachteule]

Hmmm, that's weird

Your hardware is Pi3, so Debian is used, not Raspbian

Last idea for today, are there additional files in folder /etc/apt/sources.list.d/ ?
Usually there is xbian.list only

And, please post output of

Code:

sudo grep -r mirror /etc/apt/