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You would need to send a photo in to your local College or University to their Horticultural Lab. There is no way I can say without some sort of photo or two, or perhaps three!

You could have either a thrips problem or they are not getting enough water for the temperatures you are having. A systemic insecticide is needed to really get after the thrips, so I would give them a treatment of Bayer Tree and Shrub Granules and water it in well. The systemic in the Bayer product will move up through the roots and into the roses natural juices to get after the thrips.

I would spray all of them with some Bayer Advanced Complete Insect killer ASAP, then give them all a treatment of Bayer Tree & Shrub Granules per the label instructions and water well. Then in about 4 days water them with some water that has both a root stimulator and a product called Super Thrive in it. Water them with this same water mix a couple times after the first time as well. There is still hope as long as you have green canes. Prune the back only to remove dead or badly wilted foliage.

Sometimes new growth starts out smaller and then gets larger as it ages, other times it can be a sign that the grafted rosebushes root stock is pushing up canes. Grafted roses have the part that we buy them for above ground while their root stock is an entirely different rose that is very hardy. The grafting is done to make for a hardy rose that can survive in most any climatic conditions. Sometimes the root stock gets a bit over zealous and pushes up its own canes. This robs the top part of the grafted rose that we desired of nutrients and strength and will eventually lead to its death. In such a case i highly recommend pruning out those canes with the different look all the way down into the ground, as close to the graft as possible. Seal the cut area with Elmer's white all purpose glue or the Tacky Glue you can get at craft stores.

If the spots move it could be mealy bugs and also could be mites. I would take a couple leaves with the white spots on them to your local nursery and ask them what you are dealing with. Make sure you put the specimen leaves in a plastic bag as most nurseries do not like such things brought into their establishments in an open condition. Once you have positive ID the proper action can be taken.

I unfortunately do not know much about growing roses in Spain but it sounds like the weather should be good for growing them. I grow some of the Austin roses and they are a delight. It sounds like you may be getting ahold of roses locally that are not good stock to begin with, especially if they do not look like what the labels say they should be.
https://www.gardeningknowhow.com/ornamental/flowers/roses/old-english-roses.htm