//forum.xda-developers.com/galaxy-s4/help/problems-official-update-signal-soft-t3250132/post63904021#post63904021
Have a look at this. I haven't tried yet because I have Internet out of order at the moment. If you try, let us know the result.
Thanks
Hi, have you fixed the soft keys bug? I have the same problem after the same upgrade. Samsung told me to change the screen but I find really strange that solution. Please let me know what you have done. Thank you very much.