My jabber-server fails to connect to gmail.com giving the error: outgoing s2s stream myserver.com.ua->bot.talk.google.com closed: undefined-condition (myserver.com.ua is a Google Apps Domain with Talk service enabled.)

I am using the Prosody XMPP server. It works just fine with other jabber-servers I tested so far (e.g. jabber.ru). However, when some of my clients tries to add a gmail contact to his contact-list, the subscription request lasts forever, and the Prosody gives the following sequence of messages in its log:

Oct 21 22:57:16 s2sout95897f8   info    Beginning new connection attempt to gmail.com ([173.194.70.125]:5269) 
Oct 21 22:57:16 s2sout95897f8   info    sent dialback key on outgoing s2s stream 
Oct 21 22:57:16 s2sout95897f8   info    Session closed by remote with error: undefined-condition (myserver.com.ua is a Google Apps Domain with Talk service enabled.) 
Oct 21 22:57:16 s2sout95897f8   info    outgoing s2s stream myserver.com.ua->gmail.com closed: undefined-condition (myserver.com.ua is a Google Apps Domain with Talk service enabled.) 
Oct 21 22:57:16 s2sout95897f8   info    sending error replies for 2 queued stanzas because of failed outgoing connection to gmail.com

Here for the domain name of my server I use myserver.com.ua

I found a similar problem described in this thread, but there is no detailed description of the solution there.

As for the Google services, I did have a google account where I added the domain name under question to the Webmasters tools page. However, I deleted my account long ago, so now it is unclear, how any of the Google services can relate to my domain name.

So my question is: What is the real cause of this problem (my jabber-server configuration or imaginary Google account or something else) and how can I make my Prosody server connect to gmail.com jabber service?