The solution is to 'Reset to Factory Defaults'.
This sort of problem is not uncommon on many pieces of kit, not just Zyxel
The solution is always to reset the settings to Factory Defaults.
Foe some reason the configuration data can get corrupted when firmware is flashed.
The corruption causes settings to be mangled and the code often cannot handle some of the data that has been set.
You get missing options or values that are not possible to enter via the GUI and lots of strange things.
The reset will overwrite all the data and start from a known state that should make sense.
Be aware that configurations saved from earlier versions of the firmware may not be considered valid formats in the later versions.
This is because options are changed and facilities as added/removed as the firmware evolves.
This means the configuration file format\content is changed.