Flaking indoor paintwork

You can get an anti humidor paint which lets it ‘breath’ then an aerosol spray to get the popcorn finish.

I have used it in several areas and so far it seems to have cured it, for how long who knows.

The popcorn finish is widespread unfortunately. 

When you say interior walls, do you mean inside the house as opposed to outside or do you mean the inside of external facing walls? If it is the latter then if the house has been closed up without heating for a while then this is most likely condensation which is common in single skin construction and we have had the wettest March for many years. 

The best thing you can do is wash off any mould and then let it dry out with plenty of ventilation and heat (not portable gas fires which just produce more moisture). When it is dry, knock off any blown plaster and patch repair.

If you are talking about walls which are fully internal then it could be rising damp as a dpc is rare in Spain. Again, follow the same steps, and if the house is going to be permanently occupied the problem may not come back. If it does you may want to call in an expert.

https://cano-sa.com/en/quitar-gotele-de-las-paredes/ 

It is not quick or easy and will make a lot of mess whichever way you try to get rid of the gotele.

If the house is going to continue to be closed up at times, you need to provide as much ventilation as possible, leaving all internal doors open, moving furniture away from external walls to help airflow and leaving trays of salt, cat litter or dehumidifying crystals in each room.

Both available at Leroy Merlin, the Gotele aerosol spray is about 10€ and there are various anti humidor paints you can buy. Once it’s done and dry just cover it with a coat of your normal paint you use on your walls