Your boiler is simply indicating that it is currently pre-heating itself to ensure hot water is available quickly when you need it.

Warmstart demand (warm start)

What causes this fault?

This status occurs when the Warmstart function is activated, causing the boiler to periodically fire up to keep its internal heat exchanger at a set temperature. It is a normal internal process designed to reduce the time you have to wait for hot water to reach your taps. No actual fault exists; the boiler is simply maintaining its own heat levels in anticipation of your next request for water.