Using "Find unlinked files" does not work.
Scanning the directory works fine, but when I press "Apply" it displays "Importing into Database..." without ever making progress.

The problem occurs only for PDFs. It is independent of filename and directory. It is also independent of file content, i.e. it even occurs for empty PDFs.

I am using Windows 7 and the problem occurs for both JabRef 2.11.1 and JabRef 3.0