Hi Rik and welcome to the site. That's a really pretty Calathea you have there.
Could you tell us a little bit more about where you are growing this plant? Near a window? Filtered light? In a kitchen, living room or bedroom? Any chance you'd want to put it outside for the rest of the summer?
These are tropical foliage plants that like lots of humidity and warmth. In summer when we run the a/c a lot, the humidity is low and it can be chilly for a tropical plant that likes it 90deg. or so to put on its maximum growth. They also need bright indirect light to be healthy.
Another observation from your pictures - looks like there are a lot of stems in that pot. This is very common for commercial growers to crowd a lot of plants into one pot so that it looks lush and healthy. But there really isn't room in that pot for all those plants. I'd suggest a new, bigger pot with fresh potting soil will help.
You should grow it in your kitchen or bathroom for higher humidity, and up high to keep it warmer. Be sure the a/c vents are not blowing cold air on the plant.