Enable https://www.mediawiki.org/wiki/Debugging_toolbar , visit a Flow board.
Some developers (using zend PHP not HHVM) get a JS error. Appending ?debug=1 reveals mediawiki.debug.js fails accessing this.data.log because this.data is null.
We think it's because Flow's SQL queries of binary(11) columns involve strings like "�#�����o~p". Shahyar Ghobadpour found that in his case the PHP conversion to JSON fails so the expected data structure isn't present. He has a gerrit patch https://gerrit.wikimedia.org/r/#/c/155832/