Q: iMessage Apps not working

Same here! I might add that I updated all my apps while still on iOS 9, maybe that makes the difference. They do start working when you delete the apps (from the home screen - not just the iMessage part of the apps) and re-download them from the App Store. But depending on how many iMessage-enabled apps you have and which, that can be a pain in the you know what...

Thanks for the idea to delete app and re-install. I also updated Giphy app while on iOS9.

Deleting the apps from the phone and reinstalling worked for me as well. Thanks for the suggestion.

deleting and reinstalling does not work for me, or a couple of friends who have same issues

Same here. No apps work.

My scenario fell into similar ones described on here: upgraded to iOS 10-ready apps while still running iOS 9, and then after upgrading to iOS 10 just the app's icon showed in the iMessage app extension drawer.  For me, not only did I have to uninstall the app and then reinstall it, I also had to reboot the phone and then the app showed up correctly.

I also had to sign out of my Apple ID and iMessage and then log back in

same worked for me. I did:

1. Deactivate the app in the iMessage widget board.
2. Delete the app from your phone.
3. Reboot.
4. Install the app again.
5. Open iMessage and add it to your widget board.
6. Profit.

Yup, this worked for me. Thanks!