Hi Beatriz, thanks for writing!
This is a known bug that manifested on some systems. It has been fixed a while ago. Are you using the latest version (Alpha 8.1)? (You can check by opening the About window.)
If not, you can download it here: https://animationpaper.com/download-pre-release/
If you do already have Alpha 8.1 and you still have the issue, please let me know, and I can help you further 🙂
Best,
Niels