Hi,
Probably isn't an LE issue. But what you can try...
First try to reboot all devices, the Fritzbox and the device/devices (if the TVheadend server and client running on separate devices). And use the "hard reset" method, power off all, wait, and power on. I had same issue once, when my USB tuners remains blocked after strange power issue.
Check if the issue coming from TVheadend server or client side. Install Kodi on another device (Android, PC), add and configure the TVheadend client and try if working. Or you can save the channel list from TVheadend server ( http://TVheadendServerIP:9981/playlist ) and open in VLC...
Disable the TVheadend server 4.2 addon and try to install the 4.3 version.