Reading Mode also works on my 8.1 8X. I agree with the suggestion to factory reset; I did that and only have one minor glitch with the HTC app live tile.
When I reported the app glitch to HTC and told the tech I already did a factory reset he told me to try a different procedure:
1. Turn the power off.
2. Press and hold the VOLUME DOWN button, until you see an exclamation mark. If it doesn't appear push the POWER button briefly while pressing VOLUME DOWN.
3. Then press the following hardware buttons in sequence:
VOLUME UP > VOLUME DOWN >POWER > VOLUME DOWN.
I didn't, since I uninstalled the app instead.