This is weird, we did not heard similar problem before. Could you please try to use our free software the Xkey PLUS to switch off the polyphonic aftertouch, then try it again? Or maybe you can test it with a simple MIDI monitor software to see whether there are any strange MIDI message were sent from the Xkey37? If there is no special message found, I guess maybe the aftertouch message was set to some control message in your system.
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