Sounds like lack of ventilation to me if opening the windows helped.
We have an HVAC heat exchanger thing in the loft that works wonders, but the thermostat in it blew, leaving it dead. It's a pain to find someone to repair it anytime soon.
So we have a window at the back of the house and a window in the front cracked open by around 1 cm at all times to allow a smidgen of airflow, and it's been excellent--humidity has been maintained between 45%-50%.
We also have the heating set to 21C, and any heat loss from having the windows a smidge open seems marginal. We spend around £2-£3 a day on gas heating (including heating the water).
We initially tried a dehumidifier, too, but it was constantly running and desperately struggling to get the humidity below 60%.
Determined that having the windows cracked open a smidge was cheaper and more convenient--no noisy appliance, and no water tank to keep emptying.
So, I runno if that's helpful advice, but it's a helluva lot better than whipping out the bleach to tackle any mould.
Hopefully the HVAC will be sorted in the new year. When it works it's sublime, but the loft gets too hot in summer and knackers it.