The request filtering module is configured to deny a request that contains a double escape sequence

You can use following configuration in web.config

<system.webServer>
  <security>
    <requestFiltering allowDoubleEscaping="true"> </requestFiltering>
  </security>
</system.webServer>

You can use setting in web.config:

<requestFiltering allowDoubleEscaping="true" />

but it will create some security problems because this makes your site more vulnerable to malicious URLs.
What I suggest, either you use another encryption algorithm that doesn't generate these "IIS" sensitive characters, or if you wanna use the above workaround, you need to implement proper URL/User-Input validations in order to make sure that you catch and prevent all suspicious entries.