GonrasK the problem is this rule in your CSS. I'm not sure where it comes from, it's not from Flarum itself. Maybe custom style from an extension or your own custom CSS.
.modal-dialog {
pointer-events: none;
}
It causes clicks on the modal to pass through and click the backdrop all the time.
EDIT: it's in the front.css
file, so likely not a Flarum extension.