I just hit this (with reportbug 7.5.0). I had started reportbug from a terminal inside my XFCE session, and after the GUI froze, I found this on the terminal:
Use sudo to read the kernel log? I replied "y", and sudo prompted me for my password. After giving it, reportbug worked normally. So I think the issue is that reportbug's GUI is doing I/O on its controlling terminal, and sits there waiting for a reply that never comes (because the user is looking at the GUI).