Exactly the same problem here, red, slowly flashing LED on one of my controllers:
1. I charged the controller for 3 days now, using the original charger (tested on other controller and verified to be working) and with an alternative charger (also tested and found to be working with other controller).
2. I can also start the pairing mode on the controller (start search on the PC, hold the two buttons, LED flashes blue, but it doesn’t pair…
3. I tried the reset thing where you hold all buttons apart from the bottom one, plug it in to the computer, the LED starts flashing orange rapidly, let go of the buttons, unplug the cable; no change, as soon as you turn it on again the LED flashes red slowly.
4. I even physically disconnected and reconnected the battery and there was no change.
Has anyone found a solution for this?