When you get this error message, it’s basically a warning that what you are trying to insert into the database could be malicious, so it was blocked. This is the default for ASP.Net.
If you are absolutely sure that anyone using your page is trustworthy, you could put this in your Page Directive:
You could also put it in your web.config file, in the ‘pages’ node within the ‘configuration/system.web’ node. This would make it so that all pages would not validate any input.
Also, from what I understand, in DotNet 4.0, this does not work. if you are having this problem, check out:
All Things DotNet Discussed – Winforms/ASP.Net/SharePoint/WPF