Impact of humidity on radiation readings?

Humidity can affect radiation levels, but not like you are seeing. In high humidity environments the radiation levels (from a source) may be slightly lower as the water molecules will slow down/block some of the radioactive emission.

It is possible what you are seeing/your counter is counting is Radon decay (Pb-214/Bi-214). Your counter counting more when you are using more electrical things may possibly be poor shielding on your counter, or is particularly sensitive to EMI (electrical noise), or could be a poor electrical installation. To help you a little more can you answer the following:

What kind of counter do you have/are using?
Where in the Netherlands are you? (So I can see if I can find a Radon map, and see if you are in a Radon area)
What is the construction of your home? Also the bathroom? Tiles, new or old?
Approximate age of your home?
Known state of the electrical installation?

You being below sea level should also make your background reading less than someone considerably above sea level. This is due to more cosmic rays/muons being stopped by the air. I.e. they have to travel further and not as many get through. Older houses and buildings often have a slightly higher radiation background compared to modern, newer built houses and buildings. The older materials that were essentially dug up out of the ground may be slightly radioactive due to NORM (naturally occurring radioactive materials), it's the same reason Radon is caused - from the old rocks underground.