On my iMac I run TeamViewer 12 under OS X 10.9.5.
Since I have installed TeamViewer 13 at my MacBookPro (10.12), I need to start TeamViewer 12 at the iMac twice to be able to connect to from the MacBookPro.
If I don't do so, I get the "ennabling connection" message on both sides - but nothing happens. If I then quit TeamViewer at the iMac and restart it, I am able to connect to.
Nevertheless I have no problems connecting to other clients from the iMac, even if TeamViewer has not been restarted...