Use the sensor mode required by the heating installation; do not bypass floor protection.
On the nJoy UWG5, E2 means the wired floor sensor is disconnected, defective or short-circuited. If the installation requires a floor sensor, have the installer check and repair the sensor circuit. Do not select Room mode simply to hide E2 on a floor-heating system that requires floor-temperature protection.
For a compatible installation intentionally controlled by room temperature without a floor sensor, confirm that Sensor Application is set to R (Room). In this mode the thermostat uses its built-in room sensor. F uses the floor sensor; RF controls room temperature while limiting the floor temperature.
The manual allows the sensor application to be changed in the connected app or by rerunning the startup wizard after a factory reset. Prefer the app when available. A factory reset erases personal settings: wake the thermostat, hold the side button for 15 seconds, select RESET YES with the arrows and confirm with the select/accept button. In the startup wizard, select the sensor application appropriate to the verified installation.
If E2 persists after the correct configuration, have the installer diagnose it rather than repeatedly skipping the message.
Source: nJoy UWG5 manual (Sensor, Floor Protection, Factory Reset, and Errors).