Back button stopped working? This helped me

[Rafael]

The back button quit working, most of the time. I had this problem yesterday, my first day with the new tablet..... and found a cure.....it works for me anyway.
Do not allow Google Keyboard to update to version 2.0.19133.927933a.
Version 2.0.19003.937116a works fine.

Details of discovery below.

I purchased my Nexus 7 on March 15th and it booted with 4.3....had it rooted before I allowed the 4.4.2 system update to apply.
I always turn off automatic app updates when I get a new device....this helped me find the problem.
I then applied the system update and it was still working well. Had to re-root, of course.
I had made TWRP backups before and after the system update, so I was in good shape.
Next I started removing apps I did not want, then went on to update apps I was keeping.
Next thing you know...the back button stops working for most operations.

After it aggrivated me enough, I restored the post-4.4.2 update backup and the back button worked fine.
I started working my way back to my goal-config and it kept working until my last step......updating the keyboard. Then it failed.
I removed the update and after a reboot the back button works fine. Did this twice to verify the fix.

Hope it helps others!