The High Flow Next spec sheet says it can be operated in any orientation but horizontal is preferred. If it worked fine when mounted vertically but stopped working when mounted horizontally but upside down, then perhaps horizontal upside down is the problem. A picture of the device with the cover removed is below, along with the specs. Maybe the rotor is making contact with the top of the case and cannot spin properly? I suggest inverting it so its is horizontal but not upside down. I can't see your loop but it seems to me that you could loosen the fittings a bit on each side, spin it 180°, then tighten the fittings back up. If you have to take it out of the loop to RMA it, you could replace it with a short piece of tubing that is the same length as the High Flow Next. The device is 50.9mm wide (2.00"). You could also probably replace it with a 2" or adjustable extension fitting.

Nice looking rig! Are those Phanteks D30-120mm or D30-140mm?

Why did you mount it upside down in the horizontal position? The manual says the High Flow Next does not care which direction the flow is. You could flip it 180° and have the coolant pass through it in the opposite direction. I guess you could not get to the buttons due to how close it is to the upper rad, but if it is connected via USB, you really don't need to use the buttons anyway. Can you mount it in the run from the GPU to the lower rad? That is a nice straight run and I think it would look good there assuming it does not cause the tubing to sag. If you mounted it in that run close to the GPU fitting maybe it could sit on the GPU back plate to support it (if its not too hot there).

Yes I am suggesting to spin it 180° just to test it and see if it starts reporting flow rate but I don't see why you can't just flip it around so the buttons are up (assuming you are OK with not being able to reach the buttons).

I have a High Flow Next in a test loop using soft tubing, sitting on a desk. It would be easy for me to invert it and see if it stops working. I will do so later tonight and let you know what happens.

I do not believe having the Flow Next in your current orientation is an issue, as the device allows you to rotate the text 180°. rotate.jpg
FYI, 'invert' refers to text/background colors, not orientation.

You may want to check/reseat your USB connector, and make sure you have not bent the data pin.(though the display would not work were that the case)

You mentioned 0 l/h. Is that from the display unit itself? If so, that unit might have seen its last day. If the built-in display show sensible flow rate and Aquasuite is getting 0 then, like @InfoSeeker said, telemetry data are not sent to the computer correctly. After checking USB connection, check the back of the highflow NEXT and make sure that end is also properly and securely connected.
The other day, one of my LED strips went dark. I thought the strip came loose so I checked inside my case. Turned out the RGBx connection behind my external D5 Next pump came loose (I must have inadvertently kicked it under my desk somehow).

Really nice looking rig!

If you have the extra parts, perhaps [temporarily] replacing the hard tubing where the flow meter is connected with a couple stretches of soft tubing as connections to the meter - long enough lengths so you can change the pump's orientation all over the place while the system is running. That might let you know once and for all if the issue is affected by how the pump is positioned.

Citation de "InfoSeeker"

I do not believe having the Flow Next in your current orientation is an issue, as the device allows you to rotate the text 180°.)

Citation de "Conanti"

So I absolutely would flip it if I could even just to test it. But There really isn't anywhere I can.

Citation de "Speedy-VI"

I have a High Flow Next in a test loop using soft tubing, sitting on a desk. It would be easy for me to invert it and see if it stops working. I will do so later tonight and let you know what happens.

I finally tried flipping a High Flow Next upside down in my test loop, while it was running. It made no difference in the flow rate reading at all. So I do not think yours is not working because of its orientation. Odd that it would fail after only 3 months. There is not a lot that can go wrong. I was thinking maybe a bad cable or damaged connector but you said it is showing up in AQS and you are getting other data from it. Aquacomputer usually responds to emails in a day or two. Hopefully you will hear from them soon

Great News! Aquacomputer support exceeded my expectations when dealing with a dead Octo that I bought second hand and did not test for almost a year.

iI you don't have to return the old one, after you get the replacement HFN installed, there would be little risk in taking the cover off to take a look at the internals. I don't know how difficult it is to get the cover off but I suspect removing the 4 hex-head screws in the bottom would be the first (and maybe only) step. If you get it working you could install it in the vertical position and see how the values reported by the 2 HFN compare.