Problem with setting wwwroot by session with TinyMCE FileManager

Question

I'm trying to set different values for TinyMCE through ASP .NET Session. In the web.config for File Manager the SessionAuthenticator is set up with

<add key="SessionAuthenticator.config_prefix" value="filemanager" />

So in a MVC controller I do

Session["filemanager.preview.urlprefix"] = "{proto}://{host}/edunet/";

Which works, the value is used in the same way as when I do it through the config.

Session["filemanager.preview.wwwroot"] = @"C:\Projekt\EduNet\app\EduNet.Web";

Doesn't have any effect. If I set the value in web.config like

<add key="preview.wwwroot" value="C:\Projekt\EduNet\app\EduNet.Web" />

It works.

The override value is set as

<add key="preview.allow_override" value="*" />

Help!