For some reasons, your .net installation is not correct.
I think all pages of your site which contains the asp.net validator control will have problems.
You can get this control work right away if you remove all validator controls of the page.
>>I just installed the dotnetlivehelp chat software (demo) and that is working properly....! This is ofcourse also .NET.
That's possible. As the product you mentioned doesn't validate all user inputs at all.
You have two solutions.
1. Quick solution.
Open ManageUsers.aspx file and remove all validator controls.
(This will work, but you should validator the user input)
<asp:RequiredFieldValidator id="RequiredFieldValidator1" runat="server" ErrorMessage="Username is required"
ControlToValidate="UsernameBox" Display="Dynamic"></asp:RequiredFieldValidator>
<asp:RegularExpressionValidator runat="server" ControlToValidate="UsernameBox" ErrorMessage="Please keep Username one word with no spaces or special characters."
ValidationExpression="^[a-zA-Z0-9\._-]+$" ID="Regularexpressionvalidator1"></asp:RegularExpressionValidator>
2. Fix your server
asp.net Chat http://cutesoft.net/ASP.NET+Chat/default.aspx
Web Messenger: http://cutesoft.net/Web-Messenger/default.aspx
asp.net wysiwyg editor: http://cutesoft.net/ASP.NET+WYSIWYG+Editor/default.aspx
asp wysiwyg html editor: http://cutesoft.net/ASP
asp.net Image Gallery: http://cutesoft.net/ASP.NET+Image+Gallery/default.aspx
Live Support: http://cutesoft.net/live-support/default.aspx