You'll need to edit one value in your registry to go back to the old interface. You need to navigate in your registry to "HKEY_CURRENT_USER\Software\TeamViewer" and change 'UseNewUI' value from '1' to '0.' Remember to either have Teamviewer shutdown or restart it. Don't bother with TryNewUICount. That value is just the notification that a new interface is available. If you set that back to '0' then you'll see the notification you can change it to the new interface, but leaving it at '1' keeps it from appearing.
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