Send Test and Next not clickable (greyed) due to missing value in reply_id field (i think)
Not sure if this is the answer, or a better identification of the problem.

I see that in Headers, Footers etc Auto-responder ID is '8' but is not set as default.

Not sure if setting it to default, and saving and reopening my mailing resulted in button being clickable or if that was the one I edited in DB.

Will have to look in to whether someone turned that default off or it is was a minor glitch in the upgrading.