I had similar problem to those mentioned -i'm starting from an unlocked rooted 1053 de. I flashed back to stock recovery, then it kept 'almost' updating but reverting to 4.4.4. I was really dreading a clean reset, so on a whim I decided to go ahead and mfastboot the system to clean it up. After that, smooth sailing. I'm guessing that ANY difference from the stock system file will make the OTA error out. I have never used Xposed, but did have adblock which I think made minor additions to the system folder. anyway, this worked for me so i wanted to share. good luck!
thanks, flashing stock system.img did the trick in the end, finally on 5.1.