Plastic windows crack in sunlight / thermal expansion

that is just the common system. Apparently it has already been installed "1000000x" and has never caused any problems. could you upload a picture, I really can't imagine how it is with you. thanks

This is what it looks like for us

Hello,

thank you very much for the picture. I haven’t seen it like that before either. The mounting is then on the wall instead of the frame. Apparently makes more sense

You say the rail is screwed to the frame. Maybe it helps to slightly loosen the screws and thus relieve the tension in the structure?

Cause: botched installation

: yes, the screw connection was already loosened when the white covers were screwed on. doesn’t really help.