Visual Studio 2010 Load Test Errors

I setup a web performance test and it runs fine. But when I run it in a load test scenario, all the runs fail. I don't understand why, I'm getting all kinds of errors like:

00:03:10 TestList Scenario1 {POST} Exception WebTestException Context parameter '$HIDDEN1.ctl00$ctl00$PagePlaceholder$Body$Header$hdnKey' not found in test context Stack -

Or开发者_高级运维:

Extraction Rule Error ExtractHiddenFields 3 No hidden fields were found in the response.

Why is that?

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It is likely that the server returned an error page or a page that is significantly different than the testing tool expected. The tool then failed to find some dynamic data on the page that it needed for a future page - and generated the errors.