did you create files or folders from outside DNN (using RDP or FTP)

Make sure, no file or folder name contains an apostrophe ( ' ) character.

glad you solved it

Hello Sebastian
I have the same problem that the Filemanager in the Admin menu generates an Exception. In the same dnn I have two other sites which works properly and there is no problem with the File Managers. My dnn-installation is 5.1.4.
I have tried to set the permissions on the Admin page and the Filemanager module. I've dropped the module and created a new one without any luck. I compared all three sites and checked the following tables: tabs, tabpermissions and tabmodules
Everything seems to be correct.

I even checked the file permissions. Network_services has Full permission to the Portal folders.
There are no file and folder names containing '.
If this a file synchronisation issue then I don't know how to synchronise the files without accessing the File Manager.

Here is the Error message:
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 DotNetNuke.Modules.Admin.FileManager.FileManager.get_HasPermission(String permissionKey) at DotNetNuke.Modules.Admin.FileManager.FileManager.Page_Load(Object sender, EventArgs e) --- End of inner exception stack trace ---