Just wanted to post this for anyone using aplus.net and getting the "could not open socket error." After opening a ticket with aplus.net they told me I had to provide the IP address and port numbers for them to make a firewall exception.
You can find the URL's of the recaptcha servers in the serendipity_event_recaptcha.php file in the serendipity_event_recaptcha folder. Then I searched Find IP on the web and used one resolve the URL's to the IP and ports. There were three of them so I sent them all to aplus.net.
The reCaptcha works perfectly now for comments and the guestbook. Aplus.net took 48 hours since my request had to go through three tiers of techs.
~Ed587