Check your xml files with head element, as it seems some of them have characters which Magento does not allow / recognize https://magento.stackexchange.com/a/197822/3566
To avoid problems like this, you can use Sublime Text editor https://www.sublimetext.com/, with trim_trailing_white_space_on_save parameter enabled
http://docs.sublimetext.info/en/latest/reference/settings.html