The IP blocking system at is no longer working, after Memcache was installed.
The site was also moved to SSL/https recently, which included the normal .htaccess redirects (non-www to www, http to https etc).
Not sure if that is causing the issue.
The IPs are still listed as blocked at /admin/config/people/ip-blocking
But they're not being blocked and are flooding the log with 404 errors.