Hi,
You can try these:
1.
SSH to RPi, stop tvheadend service "systemctl stop service.tvheadend43".
Make a backup of your tvheadend server addon (/storage/.kodi/addons/service.tvheadend43 and /storage/.kodi/userdata/addon_data/service.tvheadend43 - use ex. filezilla).
Start tvheadend in first run mode SSH: "tvheadend -C"
Try to use the web interface (without login - first run mode) to create new user with full admin rights.
Try to login with new admin user, if working, log out, SSH: close with control+c and reboot.
Check again if the new user working.
2.
Uninstall the tvheadend server addon, when asked keep the addon data (with all your settings).
Reboot and install again.
Try to use the web interface without user (at first run should be active without any login).
Create new admin user with password. Reboot, check.
About "transfer sat-channels & (auto-)timers from 4.2 to 4.3"... check the folders in /storage/.kodi/userdata/addon_data/service.tvheadend43/
"input", "channel", "bouquet"... don't know if the 4.2 data is compatible with 4.3 version....