Sony’s “My Xperia Theft Protection” is a great feature to have because it protects your device from being stolen and wiped clean for resale. The new version of MXTP (on the Xperia Z3+, Z4 Tablet, C4, and M4 Aqua) is actually quite advanced, including bootloader-level security to best protect your device. Unfortunately, this has consequences for those who like to unlock their bootloaders.
If MXTP is turned on, you will no longer be able to unlock your bootloader. But that’s an easy fix, just turn it off and unlock away. The problem comes about when you enable MXTP with an already unlocked bootloader. Without warning, it will break your device. There is no known fix, so seriously avoid doing this.
Sony is now aware of this issue and has posted warnings on its bootloader unlock website, but no warning exists on the device itself. Basically, do not enable MXTP if your bootloader is unlocked and you’ll be fine.