There are two separate problems here.

The humidifier may or may not be keeping the house at the designed humidity..

Some humans in the house have symptoms or discomfort that may or may not be related to low humidity.

If there is condensation on the windows, normally that means the humidity is too high. If that condensation is liquid (a slight haze up to actual drips) then there is no question. That level of humidity is dangerously high. It can lead to mold growth which can lead to respiratory systems in sensitive people.

If that condensation is beautiful frost crystals, there is some chance humidity is not too high, but the window is too cold. That's bad too because you're wasting energy, but the humidity might not be too high. There is still almost no chance the humidity is too low, IMO.

How tight the house is makes a difference, too. By tight, I mean how much outside air sneaks in. That outside air is cold, so it can't hold a lot of moisture. When it warms up, it can hold more, so the air is relatively dry. Houses in the US are usually not very tight. When we lived in Germany our house was very tight and had to be ventilated by opening windows daily even in winter.