To add tracing to legacy ASMX service for detailed trouble shooting, add the following to the web.config:

<system.diagnostics>
    <trace autoflush="true" />
    <sources>
    <source name="System.Web.Services.Asmx">
    <listeners>
	      <add type="System.Diagnostics.DefaultTraceListener" name="Default">
	    	<filter type="" />
	      </add>
	      <add initializeData="local.log" type="System.Diagnostics.TextWriterTraceListener"
	    name="AsmxTraceFile" traceOutputOptions="LogicalOperationStack, DateTime, Timestamp, ProcessId, ThreadId">
	    	<filter type="" />
	      </add>
    </listeners>
      </source>
    
    </sources>
    <switches>
    	<add name="System.Web.Services.Asmx" value="Verbose"  />
    </switches>
</system.diagnostics>