Daft Mailings are not saved

The reported bug was observed somewhere int the 4.7.x
Currently I observe this issue in 4.7.8 and 4.7.9 (Different Websites)
On both sites mailing worked previously (not sure if this was in 4.6x)

Setting: Drupal 7.50
PHP Version 5.6.23

As soon as a new mailing is started an error message "Sorry an error occurred and your information was not saved" keeps coming up (see screenshot) and no further actions can be taken.

Sometimes when starting a new mail it can be saved a few times but than again the error keeps popping up.

Nevertheless the basic mail appears in "Draft an Unscheduled Mailings"