Hello:  We were having this issue in moodle 2.7.+ and then we upgraded yesterday to 2.9.+ latest and I am still experiencing this, so I'm thinking it's probably a setting?  We have only one extra filter/plugin installed, which is a wiris math module... so pretty basic moodle install running the 'clean' theme.

I can open a kids quiz, click on 'override mark' for a question, and when I put in an answer and hit 'save', nothing happens?  any ideas why?

I even traced it out in my web console to see if there were errors, but nothing, the form will not submit and the popup will do nothing?