Hi Rene,
Please accept my apologies for the inconvenience.
It looks like your media file names contain a special character.
Please go to Admin dashboard > WP Security (main menu) > Settings (submenu) > General Settings (First tab) > Disable All Firewall Rules and press on the “Disable All Firewall Rules” button. It will cause disable all firewall rules.
Let me know if your problem will be resolved your issue.
If it resolves the issue, please enable them one by one firewall rule and check the latest enabled firewall rule has not created an issue. If you find the problematic firewall rule, please retain it as disabled.
Hope it helps you.