Okay, so after i added the App_Code and IFrame folders, I got that error above. I found a solution on another forum where I add the following to my web.config file:
<pages validateRequest="false" enableEventValidation="false">
<namespaces>
<remove namespace="System.ComponentModel"/>
<remove namespace="System.Data"/>
<remove namespace="System.Data.SqlClient"/>
<remove namespace="System.Drawing"/>
<remove namespace="Microsoft.VisualBasic"/>
<remove namespace="System.Globalization"/>
<remove namespace="DotNetNuke.Services.Localization"/>
<remove namespace="DotNetNuke.Entities.Users"/>
<remove namespace="DotNetNuke"/>
<remove namespace="DotNetNuke.Common"/>
<remove namespace="DotNetNuke.Data"/>
<remove namespace="DotNetNuke.Framework"/>
<remove namespace="DotNetNuke.Modules"/>
<remove namespace="DotNetNuke.Security"/>
<remove namespace="DotNetNuke.Services"/>
<remove namespace="DotNetNuke.UI"/>
<remove namespace="DotNetNuke.Entities.Portals"/>
<remove namespace="DotNetNuke.Common.Utilities"/>
<remove namespace="DotNetNuke.Services.Exceptions"/>
<remove namespace="DotNetNuke.Entities.Tabs"/>
</namespaces>
</pages>
<httpModules>
<remove name="Compression" />
<remove name="RequestFilter" />
<remove name="UrlRewrite" />
<remove name="Exception" />
<remove name="UsersOnline" />
<remove name="DNNMembership" />
<remove name="Personalization" />
<remove name="ScriptModule"/>
<remove name="Analytics"/>
</httpModules>
<httpHandlers>
<!-- This is for CAPTCHA support -->
<remove verb="*" path="*.captcha.aspx" />
<!-- This is for Serving files, secure, insecure, from database -->
<remove verb="*" path="LinkClick.aspx" />
<!-- This removes syndication support -->
<remove verb="*" path="RSS.aspx" />
<!-- This removes legacy support for the Logoff page -->
<remove verb="*" path="Logoff.aspx" />
<!-- ASP.NET AJAX support -->
<remove verb="*" path="*.asmx"/>
<remove verb="*" path="*.asmx" />
<remove verb="*" path="*_AppService.axd" />
<remove verb="GET,HEAD" path="ScriptResource.axd" />
<remove verb="POST,GET" path="ajaxpro/*.ashx" />
</httpHandlers>
And this worked for me. Not sure why exactly, but it worked.