We have reports that older versions of Kopete have trouble connecting to jabber.org after the server migration. These issues do not affect Kopete on KDE 4, only on KDE 3. Kopete 3.x does not support STARTTLS (encryption upgrade on port 5222) and right now jabber.org does not support the old SSL-only port 5223, so you will need to disable SSL connections via port 5223 and connect unencrypted on port 5222. Unfortunately that is *not* secure and we do *not* recommend unencrypted connections (in fact we might forbid unencrypted connections in the next few months). However, we also have reports that even unencrypted connections are not working for all KDE 3.x users. We are working to debug the problem and will post again once we have determined the cause. In the meantime you might consider upgrading to KDE 4 (yes, we know that is a big upgrade) or using another Linux or cross-platform client, such as Gajim or Psi. We apologize for the inconvenience.
UPDATE: It seems that there are issues with KDE 4, too. We are working to isolate the problem and will report back as soon as possible.
UPDATE #2: A fix is in place for a server-side bug here, and we have reports of successful connections using versions as early as Kopete 0.12.