I'm using the free version and have always been able to connect to my mother's Windows Vista desktop which is running Teamviewer Version 14.2.56676 and I am running Windows 10 with Teamviewer Version 15.12.4 (32-bit).
It stopped working saying that the Vista machine is running an old version of Teamviewer that needs to be updated. When we try to update to the newer version on the Vista machine it gives an error message.
Help. My mother will love you.