I ran into this problem and fought with it, currently seem to have it sorted.
Compare Google TTS Authentication: Error 400: redirect_uri_mismatch - #7 by rubens
One caveat remains: when using textual config for googletts I had to re-authorize after each openhab restart, probably cause the Authcoderemained in the config file but is used up after the authorization. thus I returned to configuring this via UI.