This happened to my R40/X20, resetting with a pin which typically fixed most problems didn't work - but if you examine the manual, it says red light meant HEPA filter not installed - which is a big clue - it turned out the filter was installed (by she who shall remain nameless ;-) ) wrong - essentially 6 o'clock of filter was placed at 12 o'clock position - not difficult to do by pushing it in, not fitting the small indent of the machine by the small protrusion on the filter. Clearly the machine is clever and monitors the position of the filter protrusion. Worked first time once corrected. Not sure if Battery Doc checked this, it takes 2 seconds to do. Hope it helps!

Did someone ever changed a single battery? Is there a video how to change them? I can't understand why I can find a Dyson battery for 20$, but a X20 battery pack it costs 80$!

You can't use a vacuum years after years without a thoroughly cleaning. I can see how much dust there are and i can only imagine how much dust there are inside it ...

The cells are good. It could be the 10k NTC thermistor. I fixed a red and blue light issue with a Roidmi F8 before with the help of this video: kzfaq.info/get/bejne/Y7CBh7pktp-3qGw.html It's in Italian, but it was very useful to me.

Had issue. Magnet in filter had fallen out. So it thinks canister isn't there

maybe motor clogged?

But you know, that u have to press and hold the power button for 3 seconds till the vacuum turns on?🤔