POPcon runs a self-test on each blacklist server at startup. This error means the blacklist server is returning a positive (spam) result for a known-clean test address. POPcon disables the blacklist to prevent all your mail from being tagged as spam.
This is almost always caused by a DNS misconfiguration where your DNS server appends extra suffixes to queries, making unrelated IP addresses appear to be listed.
Fix
Check your network adapter's TCP/IP Advanced DNS settings and remove any extra DNS search suffixes. Then restart POPcon to re-test the blacklists.