The SPPB component does not allow uploading images, if a user is the creator of the page.
Only when the user belongs to the Manager group (Editor is not sufficient) can images be uploaded. And then, off course, the user does not need to be the creator of the page.
My aim is to restrict the editing rights to pages of which the user is set to be the creator.
This problem occurred after upgrading from version 3 to version 5 of SP Page Builder.