Your boiler has temporarily stopped working because it has detected that water is the not moving through the system quickly enough to safely carry heat away.

Appliance is within the waiting period of the operation block function due to water shortage (VL- RL spread too big)

What causes this fault?

This fault occurs when the boiler's internal sensors detect that the heat being produced isn't being carried away by the water fast enough, causing a rapid temperature spike. The most common reasons are a failing pump that can't push the water around or a blockage caused by a buildup of sludge and debris within the system. It can also be triggered if air is trapped in the pipes or if the radiator valves are all closed, preventing any flow.