A lot of people ask for help and they don't give enough information to us to help them. Don't ask for help without an error report. You must enable log_errors=on and get the PHP error from the apache error log. Just posting: "An error occurred on the webserver and supplied invalid JSON data. Contact your system administrator or check the webserver error log." is not enough. When this happens, an error on the webserver occurred and you should look in the apache error log to find it.
Also tell us which Group-Office version, server os and PHP version you are using.
Please don't post bugs here. Use the Sourceforge bug tracker here: