Keurig K40 Add water+heating+medium cup flashing

I have a Keurig used for many years. Recently, it started refusing to brew, and flashes the "add water"+heating+medium cup LEDs.

I googled online, and found many had this problem, and a workaround is to press-hold the medium cup, and wait for all lights to be solid, and then brew again. I tried it, but it still can't let it brew.

when it brews, both the jet in the coffee k-cup holder and the return line (right above the water tank, the left-outlet of the return) flow hot water. If I use my finger or a plier to block the return line, the error state can come sooner.

here is a video showing the symptoms.

https://photos.app.goo.gl/BBkXhiYvuS67v7fV8

I opened up the unit and checked all tubing connections inside, everything looks ok - I also blew some air through the water inlet to the brewer under the water tank, I see water pushing out of both return line and the hot water pin in the k-cup holder.

I am wondering if anyone knows what could be wrong?

thanks

 

Unfortunately other than cleaning.... I haven't had much luck repairing those machines.