In particular situations warning messages are displayed in back-end of Joomla!®, caused by BwPostman. I was very willing to catch all messages, for that no messages are displayed. But in some cases there is no chance to do so, for that messages are displayed from time to time.
The warning messages occur, when
Joomla!® Error Reporting is set to Maximum or Development at System → Global Configuration → Server
and/or display_errors is set to ON in php.ini of the server