TeamViewer 12 not connecting

We have a commercial license and recently I have not been able to connect to any remote PCs.  I've tried connecting to several remote PCs that have versions 10, 11 or 12 installed and none of them connect.

When I click connect (or double-click the remote PC in the manager), it hardly ever connects anymore.  From the TV desktop manager, I can see it says Connecting ... and then tries connecting for several seconds when it finally just stops with no connection and no error.  Every once in a while, I can get a remote PC to connect that refused to connect for several attempts previously, but mysteriously goes passed Connecting ... message on to the Authenticating ... message and then finally connects.  But that is rare.  When I try to connect using the web manager, it never connects.

Somehow my copy of Teamviewer upgraded to v13 - I don't recall doing that, at least on purpose.  I'm tired of being pestered by all my programs that feel the need to interrupt me while I'm working to install an update.  I have work of my own to do!  I downloaded the latest version of v12 and installed it.  But that made little or no difference.

How can I get this TeamViewer to work consistently again like it did just about a month or so ago?  I was out sick for many weeks and when I came back, I discovered this problem.  My computer was not used by anyone else in my absence because it's in my office at home.  I need to get back to reliably connecting to the PCs we service as soon as possible!