I was trying to update my Sony Xperia Z3 Compact from Android 5.0.2 (rooted) to Android 5.1.1, and everything worked well except that I got this issue : mount: Operation not permitted when I was trying to put in write mode the system folder.
Also I tried SDFix and I got the error « Update failed, platform.xml file could not be updated ».
When I tried « ES Explorer », with Root Mode, I had SuperSU asking for root permission, but then « ES Explorer » said my system is not rooted.
After hours of digging I finally found I had to use that file discovered in that XDA thread. Just download it, unzip, then launch the install.bat