I had this same problem when iI updated my s4S4 software within. Within a few weeks iI was having white blocks instead of numbers or sometimes it was letters. ItIt also affected by pinPIN lock numbers would.
Would you believe iI read all the reviews, but in the end iI just turned my s4S4 off and back on nothing else i. I didn't reboot, but iI was happy when iI switched my phone back on and everything was back to normal (no blocks ) please. Please try it before you do anything else