OTA-7 update is here on my settings tab. Yeah!
After pushing the "Install" button the device reboots into recovery, shows the "installing updates" screen and finishes in the recovery menu.
After rebooting the version is still OTA-6 and the settings still shows the pending update. Not so yeah!
As I was writing this the typical "It can't be that petty thing" thought crossed my mind. And yes, it seems like my soft link from /var/cache/apt/ to /userdata/.... was inhibiting the update.
So I undid the link, restarted the installation, and now everything works fine.
I didn't retract my post, because there might be more wise guys like me who did the same thing and might run into the same problem.