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Assistance with Gas Heater - Brivis HPS HE20 External - Error Code 503_FAILS

I'm experiencing an issue with our Brivis central heating system and would appreciate your guidance. Model: Brivis HPS HE20 External Error Code: 503_FAILS > PRESS RESET When the unit is reset, it always starts up successfully on the first attempt and runs without any issues. However, once it's turned off, it fails to restart and shows "ERROR" on the control panel. Please let me know: - If there’s a quick or recommended reset procedure, I can try myself. - How best to proceed if the issue persists. I’m attaching a few photos of the unit and the model label, and can send more if needed - just let me know. Thanks