After Effects CC 2015.3 is a separate install from CC 2015 - think of it as CC 2016 because that's basically what it is despite some marketing wonk at Adobe who decided to call it otherwise. (Be aware that the CC app defaults to removing old versions when you install new ones, so it can seem like it's just updating your version vs. installing a whole new one.)
So you'll need to install your scripts in CC 2015.3 just like you did in CC 2015. Your folder structure should still be where it was. When the CC app uninstalls the old version, it leaves some of this kind of stuff behind. So you should be able to simply copy stuff across to the new version.
I had the same problem here, and for me, the thing that sove this problem was a simple rename on the After Effects directory.
(from Adobe After Effects CC 2015.3 to Adobe After Effects CC 2015)
I hope that sove for you too.
I was able to contact the script creator and it turns out that the problem was with the preference file.
After I reset my preferences everything was good to go! If you have a script that is correctly installed and
you are still getting errors try resetting your preference file.
Yep, that worked for me. Also happened with CC17.