Although I dont know the Boema in detail.... in general
The level probe (when water is low) operates a relay which....
1. opens a solenoid valve to allow water from the pump to flow into the boiler
2. supplies power to the pump to fill the boiler.
When the probe again detects water in the boiler at the correct level, the relay drops out....
Does the pump operate when water is low? I suspect the water low circuit isnt operating at all..... and as you can rule out the pump (as you can brew) it is probably the relay which is faulty.
If the pump operates but the boiler doesnt fill.... then Id suspect a faulty boiler fill solenoid.
Hope that helps