Need to add to this, as i thought i was going mad. Sometimes it is clickable, mostly not. I do not know what the circumstances are that changes this. I thought that maybe it was due to a question with JS included. but problem still there after removal.

Added 2 new screen grabs. one with red arrows showing the error states, the second as it should look. To get it corrected you reload the page and it corrects.
This seems to be a similar problem to the one I have reported some time ago to do with the set conditions tab frame. (13940)

I am using the orange admin theme, so for clarity, in the first image with the arrows everything that is blue should actually be orange, the second image shows this corrected after reload. please ignore the images in post at 18:59 the ones at 19:00 are correct.

Further update. Looks like I can replicate the colour issues by just selecting questions (anyone) in the left question list pane. however the "Add new question" issue remains ie sometimes clickable sometimes not - does not seem to have a pattern.

Still unable to use the Add New Question in the right pane (under the questions) see first image.