Actually, I imported a working form (xml file) and just modified it.

Hi @Jack2019 

It looks like you have a ":" specified in your URL which is not supported. Can you double check your URL's if you are constructing them and remove that character?.  This can also happen on the Redirect URL's.

Regards,

This error is because you have HTML tags in your POST request . It can be an indication of a cross site scripting attack, which is why Asp.net does not allow it by default. So, you should encode at the point where some specific characters may become dangerous because they cross into a different sub-language where they have special meaning.

The solution for this error is that you should either HTML encode before submitting , or Add the following to the IIS web.config file:

<system.web>
<httpRuntime requestPathInvalidCharacters="" requestValidationMode="2.0" />
<pages validateRequest="false" />
</system.web>

The file is located in C:inetpubwwwrootweb.config.