I think I am having a similar problem... I have 4.0.2 dotnetnuke and these are the errors I get when i try nad open the file manager from the admin menu

 

An error has occurred. DotNetNuke.Services.Exceptions.ModuleLoadException: Object reference not set to an instance of an object. ---> System.NullReferenceException: Object reference not set to an instance of an object. at System.Web.UI.UserControl.get_Request() at DotNetNuke.Entities.Modules.PortalModuleBase.get_IsEditable() at DotNetNuke.UI.Containers.Title.CanEditModule() at DotNetNuke.UI.Containers.Title.Page_Load(Object sender, EventArgs e) --- End of inner exception stack trace ---

Unhandled error loading module. DotNetNuke.Services.Exceptions.ModuleLoadException: C:\Websites\dnn4EclipseTouring\Admin\Files\FileManager.ascx.vb(709): error BC30560: 'ZipEntry' is ambiguous in the namespace 'ICSharpCode.SharpZipLib.Zip'. ---> System.Web.HttpCompileException: C:\Websites\dnn4EclipseTouring\Admin\Files\FileManager.ascx.vb(709): error BC30560: 'ZipEntry' is ambiguous in the namespace 'ICSharpCode.SharpZipLib.Zip'. at System.Web.Compilation.BuildManager.CompileWebFile(VirtualPath virtualPath) at System.Web.Compilation.BuildManager.GetVPathBuildResultInternal(VirtualPath virtualPath, Boolean noBuild, Boolean allowCrossApp, Boolean allowBuildInPrecompile) at System.Web.Compilation.BuildManager.GetVPathBuildResultWithNoAssert(HttpContext context, VirtualPath virtualPath, Boolean noBuild, Boolean allowCrossApp, Boolean allowBuildInPrecompile) at System.Web.UI.TemplateControl.LoadControl(VirtualPath virtualPath) at System.Web.UI.TemplateControl.LoadControl(String virtualPath) at DotNetNuke.UI.Skins.Skin.InjectModule(Control objPane, ModuleInfo objModule, PortalSettings PortalSettings) --- End of inner exception stack trace ---