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We are having the same problem. It looks like a ventilation issue and we've called in a specialist who seems to be recommended - Humihouse.
You need water somewhere it shouldn’t be for condensation, so some quick checks. If you have a tumble dryer, check the venting tube is connected and not blocked - happened to me last month. Check venting in boiler room, from your cooker hood and go round the house with a torch checking for damp walls.
In the meantime buy a dehumidifier to get the humidity in control.
There is a high risk of respiratory infection with high humidity levels - once the mould spores start to grow it will be difficult ..
So a dehumidifier is worth the investment .
As it seems the problem is for the whole building try to act together with some or all your neighbours, more effective and costs are shared.
Experts bill heavily..