Re: Connection Could not Be Established. Reason Unknown.
Teamviewer Support suggested a complete uninstall and reinstall:
1) Uninstall TeamViewer from the Control Panel > Programs & Features
2) Delete the TeamViewer folder at below. C:\Program Files (x86)\TeamViewer %appdata%\Teamviewer
3) Delete the TeamVewer keys in the Registry: HKEY_LOCAL MACHINE -> SOFTWARE -> TeamViewer [32 bit OS] HKEY_LOCAL MACHINE -> SOFTWARE -> Wow6432Node -> TeamViewer [64 bit OS] HKEY_CURRENT_USER\Software\TeamViewer (*Deleting other important keys in registry may cause a serious problem to your computer.)