upload file button – nothing happens

I have just created a new wordpress page and this plugin has the problem that nothing happens when I click on the “upload file” button.
In my test installation of wordpress, everything works fine.
when I click on the “upload file” button the dialog for uploading a new document or selection from the media library appears.

can someone give me a hint what this could solve?
if i manually enter the url to a document from the library it will work.