The linked article doesn't help. Don't bother with factory reset. You will see a temporary solution and the problem will come back a day later.
Wiping the cache partition also doesn't help.
This year we are experiencing how HTC destroyed its flagship. I so loved my M9, its design, the BoomSound speakers, Blinkfeed..
What did HTC do?
First they delivered a firmware that wasn't ready. Overheating, bad camera quality..
Then they solved most camera issues (Kudos to them) and destroyed the way the device charges. That wasn't enough though, they brought ads to BlinkFeed, my favorite feature of Sense.
Goodbye HTC. I hope to see you again when you start making sense again. You are the disappointment of the year