Ok file this one under weirdes bug evar cause I have no idea why, but adding the event calendar module to my SS installation (running on branches/2.4) empties the save button in the Member popup form. I was tearing my hair out about this and finally tossed out all my modules and it suddenly worked again. After adding them back one by one I found that when adding event_calendar save button in the member popup goes blank.
Like I said I have absolutely no idea why. I'm going to try and figure that out now.