I am having an issue with any media item over the size of around 10 mb. In the media section after creating a new file and hitting save the following messages comes up in a red prompt.

    Request error: The URL returned a 404 (not found): 
/umbraco/backoffice/UmbracoApi/Media/PostSave
with data: 
{"key":"contentItem","value":{"id":0,"properties":[{"id":0,"alias":"umbracoFile","value":{"selectedFiles":"LargeFile.pdf"}},{"id":0,"alias":"umbracoExtension","value":""},{"id":0,"alias":"umbracoBytes","value":""}],"name":"test larger file upload","contentTypeAlias":"File","parentId":5019,"action":"saveNew"}}
Contact your administrator for information.

Now we have gone through all of the suggestions in several threads about increasing the maximum upload size like this one.

For reference the Web.Config has been increased in size to support files up to 512 mb.

<httpRuntime requestValidationMode="2.0" enableVersionHeader="false" targetFramework="4.5" maxRequestLength="524288000" executionTimeout="99999" fcnMode="Single" />

<security>
  <requestFiltering>
    <requestLimits maxAllowedContentLength="524288000" />
  </requestFiltering>
</security>

Essentially I am coming up empty with other ideas. Nothing is being thrown in the Windows Event Viewer and nothing is showing up in the Umbraco Trace logs. The error message is thrown near instantaneously after hitting Save. Smaller files work perfectly fine and never encounter the issue. I am hoping for any thoughts that the community might have.