For IIS6 maximum allowed request length is 4096 in KB, which is about 4MB. By changing httpRuntime -> maxRequestLength should do the trick. More info: http://support.microsoft.com/default.aspx?scid=kb;EN-US;295626

Can you post fragment from web.confg?

<system.web>
<httpHandlers>
<clear />
<!-- This section is copied from root web.config -->
<add path="WebResource.axd" verb="GET" type="System.Web.Handlers.AssemblyResourceLoader" validate="true" />
<add path="*.aspx" verb="*" type="System.Web.UI.PageHandlerFactory" validate="true" />
<add path="*.ashx" verb="*" type="System.Web.UI.SimpleHandlerFactory" validate="true" />
<add path="*.asmx" verb="*" type="System.Web.Services.Protocols.WebServiceHandlerFactory, System.Web.Services, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" validate="false" />
<add path="*.ascx" verb="*" type="System.Web.HttpForbiddenHandler" validate="true" />
<add path="*" verb="*" type="EPiServer.Web.StaticFileHandler, EPiServer" validate="true" />
</httpHandlers>
<httpRuntime maxRequestLength="20480" executionTimeout="1200" />
<pages>
<controls>
<add tagPrefix="EPiServerUI" namespace="EPiServer.UI.WebControls" assembly="EPiServer.UI" />
<add tagPrefix="EPiServerScript" namespace="EPiServer.ClientScript.WebControls" assembly="EPiServer" />
<add tagPrefix="EPiServerScript" namespace="EPiServer.UI.ClientScript.WebControls" assembly="EPiServer.UI" />
</controls>
</pages>
<globalization requestEncoding="utf-8" responseEncoding="utf-8" />
<authorization>
<allow roles="WebEditors, WebAdmins, Administrators,StimAdmin,Kung" />
<deny users="*" />
</authorization>
</system.web>
</location>
<location path="UI/edit/CMS/admin">
<system.web>
<authorization>
<allow roles="WebAdmins, Administrators, StimAdmin,Kung" />
<deny users="*" />
</authorization>
</system.web>
</location>
<!-- Uncomment the following section in order to enable Office integration on the application. -->
<!--<location path="UI/OfficeIntegration">
<system.web>
<authorization>
<allow users="*" />
</authorization>
</system.web>
</location>-->
<location path="WebServices">
<!--
Configure the EPiServer.Security.BasicAuthentication module to send a basic authentication challenge
instead of a redirect to the forms login page. For this to take effect the EPiServer.Security.BasicAuthentication
module must be added to the list of http modules.
-->
<episerver.basicAuthentication sendBasicChallenge="true" basicRealm="" />
<system.web>
<httpRuntime maxRequestLength="20480" executionTimeout="1200" />
<authorization>
<allow roles="WebServices,Administrators" />
<allow users="*" />
</authorization>
</system.web>
</location>
<location path="util">
<system.web>
<pages>
<controls>
<add tagPrefix="EPiServerUI" namespace="EPiServer.UI.WebControls" assembly="EPiServer.UI" />
<add tagPrefix="EPiServerScript" namespace="EPiServer.ClientScript.WebControls" assembly="EPiServer" />
</controls>
</pages>
<globalization requestEncoding="utf-8" responseEncoding="utf-8" />
<httpHandlers>
<add path="WebResource.axd" verb="GET" type="System.Web.Handlers.AssemblyResourceLoader" validate="true" />
<add path="*.aspx" verb="*" type="System.Web.UI.PageHandlerFactory" validate="true" />
<add path="*.ashx" verb="*" type="System.Web.UI.SimpleHandlerFactory" validate="true" />
<add path="*.asmx" verb="*" type="System.Web.Services.Protocols.WebServiceHandlerFactory, System.Web.Services, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" validate="false" />
<add path="*" verb="*" type="EPiServer.Web.StaticFileHandler, EPiServer" validate="true" />
</httpHandlers>
</system.web>

I've managed to get a server response now but it doesn't matter how high I set the length, it still fails.

Trying to send a form with some text values and a 6,67Mb file with a file uploader in the form

causes a "Maximum request length exceeded" server error after the upload reaches 86%.

This seems familiar. Could it be that you need to configure both maxRequestLength (under <httpRuntime>) and a matching maxAllowedContentLength (under <system.webServer>) ?

See http://www.codeproject.com/Tips/72592/IIS-7-needs-extra-configuration-to-allow-large-fil

This is for IIS6, it does not have system.webServer elements - http://www.iis.net/configreference/system.webserver.

Anything interesting if you enable every possible logging available? :)

The solution was pretty simple...

I needed to add <httpRuntime maxRequestLength="20480" executionTimeout="1200" /> in the very first <system.web> section in web.config and not in any location specific section.

Great success :), however you do have <httpRuntime> with maximum length in first system.web in fragment you posted. Therefore I tried to "look" for other cases. Anyway great that you solved.

I see that. What isn't clear is that it's from within a location specific section. Poor copy/paste, maybe it would have been clearer otherwise. =/