i can't touch and confirm the back up my data option. i am able to pick the "do not back up" option whenever. ive tried adding a password but does not let me back up. im not sure all is update and installed. im running kk 4.4.4 on verizon s5. i downgraded from lollipop if that may be the issue? I dont see how if im already on the full back up prompt.
I actually got it to work by disabling the screen filtering app. I had applied bluelight filter which decreases the opacity of screen. Anyhow I turned it off and the backup worked. I wasn't able to download apk's so this helped. If u have any apps that run on top of most apps or something like that as it was explained in this youtube video, I would disable them and then do what I want to do and redownload. I don't know why this is happening now because it didn't before but it helped me. Hope it helps you.