I’ve been unable to connect to the jabber.org server using centericq, and after a bit of google-ing, I’ve found the reason why. It turns out that centericq sends a request for iq:agents, which is deprecated. ejabberd returns with 503 Service Unavailable, and centericq disconnects immediately. The fix for this is to simply stop centericq from sending this request. In src/hooks/jabberhook.cc comment out the following lines:

x = jutil_iqnew(JPACKET__GET, NS_AGENTS);
xmlnode_put_attrib(x, "id", "Agent List");
jab_send(jc, x);
xmlnode_free(x);

This appears to be a known issue.