This error occurred when I tried to upload skins to my portal. Happened in both Admin and Host. Normal upload in File Manager works fine.

Environment : DNN 3.3.7 - localhost

ModuleId: 361
ModuleDefId: 21
FriendlyName: File Upload
ModuleControlSource: Admin/Files/WebUpload.ascx
AssemblyVersion: 03.03.07
PortalID: 0
PortalName: My website
UserID: 1
UserName: host
ActiveTabID: 46
ActiveTabName: File Manager
RawURL: /banbe/Admin/FileManager/tabid/46/ctl/Edit/mid/361/ftype/Skin/rtab/49/Default.aspx
AbsoluteURL: /banbe/Default.aspx
AbsoluteURLReferrer: http://localhost/banbe/Admin/FileManager/tabid/46/ctl/Edit/mid/361/ftype/Skin/rtab/49/Default.aspx
UserAgent: Mozilla/5.0 (Windows; U; Windows NT 5.2; en-US; rv:1.8.1.3) Gecko/20070309 Firefox/2.0.0.3
DefaultDataProvider: DotNetNuke.Data.SqlDataProvider, DotNetNuke.SqlDataProvider
ExceptionGUID: 21d985b8-0aed-4f0d-95d2-ee5695de69b3
InnerException: No password set.
FileName:
FileLineNumber: 0
FileColumnNumber: 0
Method: ICSharpCode.SharpZipLib.Zip.ZipInputStream.GetNextEntry
StackTrace:
Message: DotNetNuke.Services.Exceptions.ModuleLoadException: No password set. ---> ICSharpCode.SharpZipLib.ZipException: No password set. at ICSharpCode.SharpZipLib.Zip.ZipInputStream.GetNextEntry() at DotNetNuke.UI.Skins.SkinController.UploadSkin(String RootPath, String SkinRoot, String SkinName, Stream objInputStream) at DotNetNuke.Modules.Admin.FileSystem.WebUpload.cmdUpload_Click(Object sender, EventArgs e) --- End of inner exception stack trace ---
Source:

Please explain to me why this happened and how to fix it?

Best regards,
Duc