Restored my phone back to 100% stock, with locked bootloader and S-ON, and guess what, Miracast works again.
So now I wonder, why doesn't it work when a device is unlocked and/or S-Offed?
Device 1: HTC One (Silver) S-On
CID: HTC__E11
ROM: Stock HTC
Android Version: 4.4.2
Kernel: Stock
Recovery: Stock