The somewhat long title is also an exact error Message. A customer experienced that some external users were not able to join their scheduled Conferences. I tried myself, and was prompted With the same Message.
I remember having read Tommy Clarke’s blog (http://www.ultimate-communications.com/2012/11/the-good-old-region-bug-is-back-in-lync-server-2013-lync/) on the problem some time ago, but at the time thought this only was related to Lync Server 2013 – and my customer runs a 2010 server.
Then I came across a blog post from Mika Seitsonen (http://translate.google.com/translate?hl=no&sl=fi&tl=en&u=http%3A%2F%2Fkilta.sovelto.fi%2Fcore%2Fwindows-8%2Fetko-voi-liittya-lync-2013lla-lync-kokouksiin%2F – English translation) pointing out the same problem that Applied for Lync Server 2010 as well.
Following the steps pointed out in their blogs the problem was fixed.