I have a Flex application that is hosted on our server and is accessed by clicking on a web tab.  The web tab points to the .html file which in turn loads the .swf.

I have a client who can't load the application.  When he clicks on the web tab he gets the following error:

(mx.messaging.messages::ErrorMessage)#0
  body = (null)
  clientId = "DirectHTTPChannel0"
  correlationId = "326CBB94-1327-9170-3836-DFD447FED461"
  destination = ""
  extendedData = (null)
  faultCode = "Channel.Security.Error"
  faultDetail = "Destination: DefaultHTTPS"
  faultString = "Security error accessing url"
  headers = (Object)#1
    DSStatusCode = 0
  messageId = "00F9E634-16FD-BADE-8F6B-DFD448BA55C1"
  rootCause = (flash.events::SecurityErrorEvent)#2
    bubbles = false
    cancelable = false
    currentTarget = (flash.net::URLLoader)#3
      bytesLoaded = 0
      bytesTotal = 0
      data = (null)
      dataFormat = "text"
    eventPhase = 2
    target = (flash.net::URLLoader)#3
    text = "Error #2170"
    type = "securityError"
  timestamp = 0
  timeToLive = 0

Client is seeing this error in both IE 8 and Firefox 4 with the latest version of Flash player installed.
 
Now, if I log into his instance, click on the web tab, I'm able to load the application just fine in both IE and Firefox and with the same exact version of Flash installed.

I can also access the application in every other salesforce instance where our applicaiton is installed.

Anybody have an idea why he's the only one getting this security error?