Some sites use Cloudflare to protect themselves against bots used in DDoS attacks. Unfortunately, this can also interfere with HDoujin Downloader. The program will do its best to get around it, but might still wind up blocked.
If Cloudflare is preventing the program from accessing a site, you can try using a proxy or VPN. This works well if your IP or IP range has been temporarily blacklisted by Cloudflare as a potential spam source. You can find information about using VPNs with HDoujin Downloader here.
Secondly, you can submit a log file containing information about the issue, and wait for a fix, if possible. If you have log files enabled (“Help > Enable Diagnostics”), Cloudflare-related log files will be written to the “log” folder in the same directory as HDoujin Downloader.