You can try and ‘descale’ the unit in a vinegar solution and run it for a few hours and see if that helps. Just like you would with a coffee pot.
From what I can tell the water level is determined by the unit through some sort of conductivity between the probes through the water under the metal shield. When it goes into the long beep when the water level is OK then there is some sort of ‘connection’ issue between the probes.
Once the vinegar solution that stopped working for me, I took a strip of aluminum foil, the length of the width of the aluminum foil box. Folded this over on its self a few times to create a 1/4" wide strip of aluminum foil. I then started by wrapping this around one of the probes, then moving to the next probe and wrapping around that one and continuing on until I connected all the probes to each other with the aluminum foil strip.
Do not wrap it around the impeller that is in the center which circulates the water.
After I did this, it seems to by-pass the water level auto shut-off (Long beep) and the unit has been running for over 2 years without the long beep issue.
Warning, if this does bypass the water level shut-off, you could damage the unit on a cook where the water level is too low.