Many thanks for your solution!
I've had the same issue with my osmc setup on the Raspberry Pi 3. (Kodi 16.1, TVHeadend Server 4.0.9)
The TVHeadend Frontend started, but didn't find any channels.
Therefore the Live TV and Radio Button in the Kodi Mainmenu didn't appear.
To solve the problem I tried to uninstall the TVHeadend HTSP Client; without success.
When I hit the uninstall button the frontend disapeared out of the list, but was back again after a reboot of the system.
After rename (or remove) the two files listed above and reinstall the Frontend everything worked like a charm!
By the way: on my setup I found the files under:
/home/osmc/.kodi/userdata/Database/