Its not those config settings causing the problem, based on the error detaill you posted its definitely IIS configuration or if you have any request filtering module installed in IIS it could be that but it is IIS refusing it not mojoPortal code.
Have you tried changing this in Web.config:
<httpRuntime requestValidationMode="2.0" maxUrlLength="560" maxQueryStringLength="2048" />
by adding a maxRequestLength attribute as mentioned in the article about supporting large file uploads:
<httpRuntime requestValidationMode="2.0" maxUrlLength="560" maxQueryStringLength="2048" maxRequestLength="2097151" />
this is an IIS configuration element that can be set in Web.config, though there may also be related things in the metabase that affect it.