You see this error as a result of action called Event Validation
This is a standard ASP.NET technique for validating requests and field
on submission. The reason is that your MenuLabel contains HTML
characters, so ASP.NET considers this as a security issue and
terminates the processing of the page. The only workaround you could
use is to disable the Event Validation, but we do not recommend it.
You should use another approach on achieving this customization. For
example, you could dynamically change the style of your menu elements
through the page's codebehind file.
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