Had the same issue on Win7 and Teamviewer 11. Funny thing was that one of five computers had connection to Teamviewer server, but the others didn't (they all worked out fine earlier). So it seemed that it hadn't anything to do with firewall. But since I couldn't find any other reason, I added a rule to firewall that allows traffic to *.teamviewer.com on TCP/UDP Port 5938. And that did the trick, now there's connection on all five computers.
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