Even if you have insulation you will still have a cold spot on the corner. The north corners are always the coldest parts of a house, hence damp from moisture in the air will gather there.
More ventilation is the answer. Dehumidifier is an overcomplicated response to a simple-ish problem. You should also clean the mould off, as the mould spores can lead to health problems.
My house is a similar construction and we get mould in teh north corners especially in the bathroom where a lot of moisture is generated.
Positive input ventilation is a good approach if ventiliation first doesn’t work, or you can keep the house warm enough when ventilated.