All of a sudden, my Amazon FPS signature string is invalid

What could be wrong? All of a sudden, I'm getting an "invalid signature string" error when I try to process payment. Here is an example of one form:

<form action="https://authorize.payments.amazon.com/pba/paypipeline" method="post">
<input type="hidden" name="abandonUrl" value="http://www.customerfind.com/canceled.php" >
<input type="hidden" name="accessKey" value="AKIAJXWUH5BVV74GEPGQ" >
<input type="hidden" name="amount" value="USD 12" >
<input type="hidden" name="cobrandingStyle" value="logo" >
<input type="hidden" name="collectShippingAddress" value="0" >
<input type="hidden" name="description" value="CustomerFind Twitter Subscription" >
<input type="hidden" name="immediateReturn" value="1" >

<input type="hidden" name="ipnUrl" value="http://www.customerfind.com/aws_ipn.php" >
<input type="hidden" name="processImmediate" value="0" >
<input type="hidden" name="recurringFrequency" value="1 month" >
<input type="hidden" name="referenceId" value="11" >
<input type="hidden" name="returnUrl" value="http://www.customerfind.com/controlpanel.php" >
<input type="hidden" name="signature" value="NmTfzEwmnzY0TT5yfD5Vh/UPxPM=" >
<input t开发者_运维知识库ype="image" src="http://g-ecx.images-amazon.com/images/G/01/asp/golden_large_subscribe_withlogo_whitebg.gif" border="0"></form>

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I think you probably should post your problem in Amazon's support center instead, they seem to know about these problems and give responses to them. Also, you might find similar errors from other users and solutions that solved their problems.

For example, after some seconds of searching, I found this entry from the last year. They also have a search engine there so you can look for posts more up-to-date.