Invited user cannot add files to the Family Folder, through the Browser

Hi, I have invited a user and shared the Family folder with him, and I want him to add files like he did when we have bought the Device.

However, he tries to drag&drop and select&upload images, but as soon as he selects the files the upload isn’t starting. There is no feedback on what is going on. He is able to create folders though.

What can we do to make invited users upload files into shared folders?

Thank you.