asp.net javascript error

We have this app that runs on over 6000 client machines. With this one customer the app throws an exception. I've traced it back to some callback javascript. I turned off their virus software thinking it was that. No luck. Any ideas on why it would be failing? See attached application error.
glenn_rAsked:
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Tom BeckCommented:
For debugging request/response there's Fiddler - HTTP Debugging Proxy. And of course there's always Firebug on the Firefox browser for debugging javascript. With Firebug you get a console for displaying errors and a watch list for monitoring variable values. No editing script though. Both are free.

30 machines on one network experiencing intermittent problems with the app? That sounds like a network problem to me. Maybe a bad switch or a bad NIC in the DNS server dropping packets occasionally. Or an IP address conflict on the network causing collisions on the data switch. They may not notice anything more than minor delays during regular web surfing. Web apps require constant server/client communication and session ids must be maintained for each client during all postbacks. That requires a reliable connection to the internet and a reliable DNS host. Don't get suckered into fixing a network problem if your only responsibility is the application.

You can download and install Wireshark on one of the 30 machines and set it to capture all traffic on port 80 (or 443 if you are on a secure socket layer) . Stress test the app during capture to see if any packets are being dropped.
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mcnuteCommented:
no error attached so far....
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glenn_rAuthor Commented:
attachment
aspneterror.png
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mcnuteCommented:
This says that the variable in the highlighted line wasn't set before using it. Set the variable with its expected content and then refresh the page.

TrainingCallbackHander =  //probably a function

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I'm not aspx expert, basically it's saying that your TrainingCallbackHandler isn't defined. So or it has no access to this function or it hasn't been created in the first place.

But this is as fare as I can go with aspx.
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Tom BeckCommented:
I don't have a solution, but it seems pointless to look for fault with asp.net when the app works as expected in over 6,000 machines. That one machine seems to be corrupting communications between server and client. Maybe a piece of malware is trying to inject something into the stream. Maybe it's just a browser setting or a malfunctioning browser. Have you tried a different browser? This is a web app, right?
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glenn_rAuthor Commented:
this customer has 30 machines with different browsers and they still get the error
also tried to disable virus protection software and same error
it happens intermittently, not all the time, they could be working and all of a sudden, error, i can induce this by having a clicking frenzy on the form and it will error out. I also went directly to the web server thinking it was latency but got the same error there. What tools can I use to debug/troubleshoot client side scripts request/response javascript callbacks etc?
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