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The solution is nearly always the same for these problems: the circuitboard under the keys needs cleaning, which will require disassembly of the unit from below. It's not a complicated job if you have a fundamental understanding of how electronic devices slot together, but as you can imagine with the keylab 88 it does require you to have lots of space.
There are various YouTube tutorials explaining how to disassemble and clean Fatar keybeds. In your case you don't need to go as far as removing the keys, just as far as getting access to the circuitboard underneath.