An update to the matter - i got it fixed.

The Reflector told me that PageReferences have been changed to ContentReferences and by changing the following statements:

        protected override bool ConvertToExternalInternal(EPiServer.UrlBuilder url, object internalObject, System.Text.Encoding toEncoding)
        {
            // We call the base implementation and if it returns true
            // then we do our magic.
            if (base.ConvertToExternalInternal(url, internalObject, toEncoding))
            {
                // The following handling rules are copied
                // from the base method.
                //PageReference pageLink = internalObject as PageReference;
                ContentReference pageLink = internalObject as ContentReference;
                if (pageLink == null)
                {
                    //pageLink = PermanentLinkUtility.GetPageReference(url);
                    pageLink = PermanentLinkUtility.GetContentReference(url);
                }
                if (PageReference.IsNullOrEmpty(pageLink))
                {
                    this.TryConvertPermanentLink(url, out pageLink);
                    if (pageLink == null)
                    {
                        return true;
                    }
                }

                // Do something if you get the PageReference - Reference is needed to determine pagetypes 

And the ContentReference needs to be converted to PageReference:

                PageData page = DataFactory.Instance.GetPage(pageLink.ToPageReference());

Hope this helps someone ;)

Good that you found a solution yourself.

But generally, using the events ConvertingToExternal, ConvertedToExternal, ConvertingToInternal and ConvertedToInternal and/or UrlRewriteModules is better since it mostly gives you less headache when doing maintenance and upgrading.