Reports fail to render with my Custom Security Extension applied - 404 error on network resources

[TestO2015]

When I try to open reports with my Custom Security Extension applied it fails to render and I see 404 errors next to certain AXD resource requests to the HTMLRenderer dll.

These resources fail to load (the rest load OK):

All resources from Microsoft.ReportingServices.Rendering.HtmlRenderer.RendererResources
https://local-PC/ReportServer/Reserved.ReportViewerWebControl.axd?OpType=Resource&Version=15.0.1102.1047&Name=Microsoft.ReportingServices.Rendering.HtmlRenderer.RendererResources.Html5Toolbar.css

https://local-PC/ReportServer/Reserved.ReportViewerWebControl.axd?OpType=Resource&Version=15.0.1102.1047&Name=Microsoft.Reporting.WebForms.Icons.SplitterHorizCollapseHover.png

I have inspected the ReportingServicesWebServer.dll where the Reserved.ReportViewerWebControl.axd handler is located and I found where it calls the Microsoft.ReportingServices.HtmlRendering.dll to get resources from the Microsoft.ReportingServices.Rendering.HtmlRenderer.RendererResources namespace. I can also see it sets the status code to 404 if it can't find the resource.

The question is what about my Custom Security Extension configuration could be causing it not to find these resources?