The boiler has lost contact with the room thermostat for one of its heating zones, so it may be heating that zone on a basic backup schedule instead of following your settings, but your home should still be heated.

DIY fix possible

Check the batteries in the wireless room thermostat (Vitotrol) and replace them if they are low or flat.Move the thermostat closer to the boiler, or check it is not blocked by metal objects, and see whether the boiler reconnects.Switch the boiler off at its on/off switch and at the fused spur for about 30 seconds, then switch it back on. Check the display for the bd code again.Check the wired thermostat connection at the boiler and the thermostat is secure with the power off. Do not open the boiler casing.If the code returns, leave the boiler on and contact a Viessmann-registered engineer, as the thermostat or its wiring may need replacing or re-pairing.