Your request must be having a < or >, thus the error.
If you cannot avoid it, then put
validateRequest="false"
in your page directive or web.config file.
Details here:
Adding Cross-Site Scripting Protection to ASP.NET 1.0[
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If you read the article, you will get the reason and why it's not suggestible.
Following few discussion, to read:
Same error discussed[
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Same error discussed - 2[
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