TeamViewer Connect to MacBook Air

I have a problem that, since a little bit, I can't have any control over other computers and other computers can't have control over mine. TeamViewer says "ready to connect". I used to have TeamViewer 11. Because of this issue; I upgrated to TeamViewer 12, but the connection still doesn't work. I shut off the firewall but the issue is still there. A few weeks ago, TeamViewer worked without any problems. A deinstalization and reinstalization didn't not help. Who can help me?