Your boiler has detected a flame when there shouldn't be one, or has a confusion in its internal electronics, causing it to shut down for safety.

Electronic fault in flame system (non–plausible flame signal) | Cause: Electronic fault

What causes this fault?

This fault occurs when the boiler's internal computer receives a signal that a flame is present even though the gas valve is closed. The most common reason is a malfunction in the printed circuit board (PCB) or a damaged sensing electrode that is sending a false electrical signal. Essentially, the boiler enters a fail-safe mode because it sees a 'ghost flame' and shuts down to ensure your safety.