just a guess, the callstack is incomplete:
DoSAttempt at 006.js calls

CloseSecureChannelHelper.Execute( { Channel: channels[i], ServiceResult: new ExpectedAndAcceptedResults( StatusCode.Good ) } );

and library/ServiceBased/SecureChannel/CloseSecureChannel.js
does this

        if( isDefined( args.ServiceResult ) ) {
            var result = args.ServiceResult.containsStatusCode( this.UaStatus );
            if( !result ) addError( "CloseSecureChannel().Result received " + this.UaStatus.toString() + ", but expected any one of the following:\n\t" + ExpectedErrors.ExpectedResults.toString(), this.UaStatus );
        }

a) this does not check if ExpectedErrors is defined
b) this should print args.ServiceResult.ExpectedResults.toString() or similar

Agreed on the fact that the object ExpectedErrors isn't defined anywhere, therefore following Bernd Edlingers suggestion.

reviewed in extra CMP call