Setting showInFileManager to false works fine in my EPiServer CMS 6 R2 demosite. Could it be a problem with any of the other VPP's registered?

Hi Mari,
your post gave me a hint and after 10 min of testing I found where is the problem and a solution for it.
Yes we are using a custom VPP. Here is the original virtualPath section in episerver.config before I found the problem

<virtualPath customFileSummary="~/FileSummary.config">
    <providers>
      <clear />
      <add showInFileManager="true" virtualName="Page Files" virtualPath="~/PageFiles/" bypassAccessCheck="false" name="SitePageFiles" type="EPiServer.Web.Hosting.VirtualPathVersioningProvider,EPiServer" indexingServiceCatalog="Web" physicalPath="SomePath\PageFiles" />
      <add showInFileManager="false" virtualName="Global Files" virtualPath="~/Global/" bypassAccessCheck="false" name="SiteGlobalFiles" type="EPiServer.Web.Hosting.VirtualPathVersioningProvider,EPiServer" indexingServiceCatalog="Web" physicalPath="SomePath\Global" />
      <add showInFileManager="false" virtualName="Documents" virtualPath="~/Documents/" bypassAccessCheck="false" maxVersions="5" name="SiteDocuments" type="EPiServer.Web.Hosting.VirtualPathVersioningProvider,EPiServer" physicalPath="SomePath\Documents" />
      <add name="App_Themes_Default" virtualPath="~/App_Themes/Default/" physicalPath="SomePath\CMS\6.1.379.0\application\App_Themes\Default" type="EPiServer.Web.Hosting.VirtualPathNonUnifiedProvider,EPiServer" />
      <add name="UI" virtualPath="~/secure/CMS/" physicalPath="SomePath\CMS\6.1.379.0\application\UI\CMS" type="EPiServer.Web.Hosting.VirtualPathNonUnifiedProvider,EPiServer" />
      <add name="UtilFiles" virtualPath="~/Util/" physicalPath="SomePath\CMS\6.1.379.0\application\util" type="EPiServer.Web.Hosting.VirtualPathNonUnifiedProvider,EPiServer" />
      <add name="WebServiceFiles" virtualPath="~/WebServices/" physicalPath="SomePath\CMS\6.1.379.0\application\webservices" type="EPiServer.Web.Hosting.VirtualPathNonUnifiedProvider,EPiServer" />
      <add name="EPiServerCMS" virtualPath="~/secure/CMS" physicalPath="SomePath\CMS\6.1.379.0\application\UI\EPiServer\CMS" type="EPiServer.Web.Hosting.VirtualPathNonUnifiedProvider,EPiServer" />
      <add name="EPiServerShell" virtualPath="~/secure/Shell" physicalPath="SomePath\Framework\6.2.267.1\Application\UI" type="EPiServer.Web.Hosting.VirtualPathNonUnifiedProvider,EPiServer" />
      <add name="Upload" virtualPath="~/Upload/" virtualName="Upload" showInFileManager="true" bypassAccessCheck="false" type="CustomVirtualPathProvider" connectionStringName="CustomVirtualPathProviderConnectionString" />
    </providers>
    <filters />
  </virtualPath>

As you can see there is one custom VPP with name Upload at the bottem of the list which inherits VirtualPathUnifiedProvider. The site was working fine, but when I changed SitePageFiles attribute from showInFileManager="true" to showInFileManager="false" the FileManagerBrowser dialog start throwing an exception.
After your post I played a bit with virtualPath section and I noticed that leaving one of the first three providers (SitePageFiles, SiteGlobalFiles and SiteDocuments) with showInFileManager="true" is enough for the dialog to work without errors. At the end I found that if our custom VPP change places with App_Themes_Default, i.e. from the end of the list to before any VirtualPathNonUnifiedProvider declaration - the dialog is working. Any declaration of others default EPiServer providers of type VirtualPathNonUnifiedProvider before our custom provider will cause an exception. So the problem should be changed to:
Declaration of a custom virtual path provider based on VirtualPathUnifiedProvider after a provider of type VirtualPathNonUnifiedProvider when SitePageFiles, SiteGlobalFiles and SiteDocuments providers are set with showInFileManager="false" will cause FileManagerBrowser dialog to throw an exception.
The solution is obvious - leave all VirtualPathNonUnifiedProvider providers declaretions after your custom VPP like this:

<virtualPath customFileSummary="~/FileSummary.config">
    <providers>
      <clear />
      <add showInFileManager="false" virtualName="Page Files" virtualPath="~/PageFiles/" bypassAccessCheck="false" name="SitePageFiles" type="EPiServer.Web.Hosting.VirtualPathVersioningProvider,EPiServer" indexingServiceCatalog="Web" physicalPath="SomePath\PageFiles" />
      <add showInFileManager="false" virtualName="Global Files" virtualPath="~/Global/" bypassAccessCheck="false" name="SiteGlobalFiles" type="EPiServer.Web.Hosting.VirtualPathVersioningProvider,EPiServer" indexingServiceCatalog="Web" physicalPath="SomePath\Global" />
      <add showInFileManager="false" virtualName="Documents" virtualPath="~/Documents/" bypassAccessCheck="false" maxVersions="5" name="SiteDocuments" type="EPiServer.Web.Hosting.VirtualPathVersioningProvider,EPiServer" physicalPath="SomePath\Documents" />
      <add name="Upload" virtualPath="~/Upload/" virtualName="Upload" showInFileManager="true" bypassAccessCheck="false" type="CustomVirtualPathProvider" connectionStringName="CustomVirtualPathProviderConnectionString" />
      <add name="App_Themes_Default" virtualPath="~/App_Themes/Default/" physicalPath="SomePath\CMS\6.1.379.0\application\App_Themes\Default" type="EPiServer.Web.Hosting.VirtualPathNonUnifiedProvider,EPiServer" />
      ...
    </providers>
    <filters />
  </virtualPath>

Hi

Nice catch. There seems to be a problem when the first non hidden vpp is not a unified vpp. I've registered it as a bug in our bug tracking system: