HTML 5 import with metadata doesn't work anymore

When I try to import documents with metadata using HTML5 D&D, it fails to import documents: nothing happens after I click Continue on the metadata form and the import popup stays displayed.

Steps to reproduce:

1. Drag a document on a workspace and wait for the area to be hightlighted in green.
2. Drop the file.
3. Click on Continue.
4. Select "Smart import with metadata".
5. Fill in a few metadata and click "Continue".
   -> The form is not displayed anymore.
   -> The popup stays here.
   -> File is not uploaded.