I tried to use wget to get images from duckduckgo using my browser's useragent, but no dice - looks like they're doing some handshaking to verify an interactive session too
May I suggest adding a note to the 'Download Cover' window that recommends adding more metadata plugins? I spent too many minutes validating the issue from my end - verifying firewall rules for calibre/calibre-parallel, etc + time spent searching for a solution until I found this thread.
Thanks!