@bmahob
Unfortunately ( or not ), Alfa commo works so that gradually reduces the force when approaching a given temperature. If you set for example to 65 C , at 62 C decreases the power to 4 ,at 63 C decreases the power to 3 , at 64 C decreases the power to 2 , and at 65 C decreases the power to 1.
And now , in your case , power in work 3 mode is not enough to heat-up water to 64 C to reduce power to work 2 mode ( heat loss in the house ).
No need to replace electronics , its all OK.
PS. :To test this - try to close one or two radiators , then water should reach set temperature.
Commo Compact 23 (4-18 kW) ,180m2 u 3 etaže na 870 mnv , izolacija 5cm zid ,20cm krov
VTC 512 @60C ,9 panelnih radijatora +termoglave , 2 sušača ,100L kombi bojler, T vode 65 C
Električni kotao Vaillant 18kW
Klima inverteri: MITSUBISHI MSZ-SF42VE, HITACHI RAC-DJ50