At Broker end, you can only do few things that you already checked: server log in MWS or logmsgs file (though to check directly logmsgs file you’ll need to know how to convert unix time to time stamp and check error code using wmgetmsg tool), diag.log (bit programmatic details, but can give you some idea).
Best is to find the error reported at client end. Anything that goes wrong will always be replied to the client making the call. So, exception at client is starting point.
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