External keyboard modifiers with Sierra stopped working (when typing non-modifier key on MacBook Pro Retina) When I upgraded to Sierra, my shift key on the external keyboard stopped working when I press a key on my MacBook Pro Retina's keyboard.
I have a setup where my left hand is using the external keyboard (to avoid pain due to chest compression), while my right hand is on the MacBook's keyboard.

Now when I press the left shift and a letter on the MacBook keyboard, the 
letter does not come capitalized. In El Capitan (and all other versions) this worked...

Any hints?

EDIT:


it's not just shift. It's command, alt, shift, control.
the modifier keys do work physically (when used with other keys on the same keyboard).
[SOLUTION!] Ha!!! Now it started working. Replying to my own question (related to Skeleton Bow's answer/suggestion - thanks) : Looks like Karabiner Elements kicked it the right way somehow. Even though I did not exactly change anything relevant, just installed it and opened it. Maybe the fact of installing /using karabiner forces sierra to use the old ways of handling modifier keys? Big thanks to @Skeleton Bow for the help in troubleshooting and for suggesting Karabiner Elements!