Occasionally I get a complaint from a customer that they were unable to make a payment - and my zencart is showing them this error -

Your transaction could not be completed due to a misconfiguration in our store. Please report this error to the Store Owner: SQ-MISCONF

Checking the relevant myDEBUG file it's getting triggered by the square_webPay module at line 235 -

trigger_error('missing token result: ' . json_encode($_REQUEST));

So why does a customer on the odd occasion have a missing token? And is there anything I can do to resolve this?