Had this problem, and repairing DCS didn't help.
In the attached file, on line 440 there is a GUI error related to logbook, so I tried deleting my logbook at ...\Saved Games\DCS\MissionEditor. This seems to have solved the problem for me: With a new logbook the button works. It is possible that at some point I have somehow broken my logbook.lua with erroneous editing.
Now I hope that the problem doesn't somehow resurface. :)
dcs_buttonerror.log