That's what I'm trying to determine.
Assuming you've tried a normal 50K pot, on the same Axoloti connections, with expected results, the only mistake I can think of is using the incorrect connections on the pot.
So, you wrote "the measurements with the Multimeter are fine", I'm guessing you didn't do that test correctly. A resistance test between the two outer pins on a pot should be constant, about 50K, regardless of wiper location. Once you've found the pins that satisfy that, the remaining pin is the wiper.
If this is obvious, forgive any offense - it's not intended. But a mistake is being made, somewhere.