adt.bat Could not generate timestamp: Cannot parse the response

I am using the command line to build an air app as follows:

adt.bat -package -storetype pkcs12 -keystore mycert.pfx -storepass xxx -tsa http://timestamp.verisign.com/scripts/timstamp.dll myApp.air myApp-app.xml myApp.swf

However, I receive this error:

Could not generate timestamp: Cannot parse the response

I am using a Verisign issued Authenticode certificate. Has anyone got any idea why this might be happening?

Thanks