Ok this is a weird one.

I have a website with DNN version 6.02.6. Everything has been working perfectly. Two days ago my client uploaded some new images, everything was fine. Later the same day, I detect some exceptions in the event viewer. The errors had to do with the fact that when pages are requested from someone, dnn searches in the cache and the access is denied for the cache file.

Today, we can't upload a single image on the website. I login with host account and try to upload an image and it fails. The image manager loads but the image is nowhere to be seen and no image pops. In the event viewer my upload attempt generates this exception:

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AssemblyVersion: 6.2.6

PortalID: 0

PortalName: My Domain

UserID: 1

UserName: host

ActiveTabID: 84

ActiveTabName: ΠΡΟΦΙΛ

RawURL: /DesktopModules/Admin/RadEditorProvider/DialogHandler.aspx?tabid=84&DialogName=ImageManager&UseRSM=true&Skin=Default&Title=Image+Manager&doid=ab3fd4aa-4061-4055-a741-6b58bc4117a3&dpptn=

AbsoluteURL: /DesktopModules/Admin/RadEditorProvider/DialogHandler.aspx

AbsoluteURLReferrer: http://my-domain.gr/DesktopModules/Admin/RadEditorProvider/DialogHandler.aspx?tabid=84&DialogName=ImageManager&UseRSM=true&Skin=Default&Title=Image+Manager&doid=ab3fd4aa-4061-4055-a741-6b58bc4117a3&dpptn=

UserAgent: Mozilla/5.0 (Windows NT 6.1; rv:44.0) Gecko/20100101 Firefox/44.0

DefaultDataProvider: DotNetNuke.Data.SqlDataProvider, DotNetNuke.SqlDataProvider

ExceptionGUID: cd853695-ca5f-45d1-87d5-69cf5a9921a4

InnerException: Unable to complete operation. An unknown error occurred. /Portals/0/Images/candle.jpg

FileName:

FileLineNumber: 0

FileColumnNumber: 0

Method: System.IO.__Error.WinIOError

StackTrace:

Message: DotNetNuke.Providers.RadEditorProvider.FileManagerException: Unable to complete operation. An unknown error occurred. /Portals/0/Images/candle.jpg ---> DotNetNuke.Services.FileSystem.FolderProviderException: The underlying system threw an exception. The file has not been added. ---> System.UnauthorizedAccessException: Access to the path 'C:\Inetpub\vhosts\my-domain.gr\httpdocs\Portals\0\Images\candle.jpg' is denied. at System.IO.__Error.WinIOError(Int32 errorCode, String maybeFullPath) at System.IO.FileStream.Init(String path, FileMode mode, FileAccess access, Int32 rights, Boolean useRights, FileShare share, Int32 bufferSize, FileOptions options, SECURITY_ATTRIBUTES secAttrs, String msgPath, Boolean bFromProxy, Boolean useLongPath, Boolean checkHost) at System.IO.FileStream..ctor(String path, FileMode mode, FileAccess access, FileShare share, Int32 bufferSize) at DotNetNuke.Services.FileSystem.Internal.FileWrapper.Create(String path) at DotNetNuke.Services.FileSystem.StandardFolderProvider.UpdateFile(IFolderInfo folder, String fileName, Stream content) at DotNetNuke.Services.FileSystem.StandardFolderProvider.AddFile(IFolderInfo folder, String fileName, Stream content) at DotNetNuke.Services.FileSystem.FileManager.AddFile(IFolderInfo folder, String fileName, Stream fileContent, Boolean overwrite, Boolean checkPermissions, String contentType) --- End of inner exception stack trace --- at DotNetNuke.Services.FileSystem.FileManager.AddFile(IFolderInfo folder, String fileName, Stream fileContent, Boolean overwrite, Boolean checkPermissions, String contentType) at DotNetNuke.Services.FileSystem.FileManager.AddFile(IFolderInfo folder, String fileName, Stream fileContent, Boolean overwrite) at DotNetNuke.Services.FileSystem.FileManager.AddFile(IFolderInfo folder, String fileName, Stream fileContent) at DotNetNuke.Providers.RadEditorProvider.TelerikFileBrowserProvider.StoreFile(UploadedFile file, String path, String name, String[] arguments) --- End of inner exception stack trace ---

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 I suspected something was wrong with the cache, so I cleared the cache, recycled the application pool, stopped the site, deleted the cached files from cache> Pages and still nothing is fixed. I get countless exceptions at the moment for denial of access in cache files which don't even exist. Permissions are all as they used to, nothing has changed and I don't know what to do at this point.

Hello and thank you for the reply.

Here is what happened in the end:

The permissions have been unchanged for two years (and everything has been working perfectly until today), so even though it seemed like the logical thing to check first, I thought this isn't the issue. There was no virus scanning at the time. The server is working normally, no backup in process. I have other sites on this server which are normally working and there are no fishy indications in the server (high cpu or something). 

Still, I checked the other sites I have on the server to make sure no other had this problem and indeed they have no issues. So I wrote off the probability of it being a server issue.

So, I did the next logical thing: I've put my trust in Sebastian. Thus I checked the IIS permissions. With terror I saw that the the IIS has only view permissions in the http folder site. And now is the next question.

I haven't changed these settings, I'm sure about that at least! Yesterday though, I gave an ftp account to my client with read-write permissions to http folder. My client gave the account to the person who is supposedly responsible for managing google analytics etc. I know he used filezilla client. Is it possible to change folder permissions through filezilla client?? I can't figure a way to do so. I searched the internet and it says it can happen  in apache but not it in IIS. Still, I don't believe in coincidences. One day I give the ftp account and the next I have this problem?

*I changed the permissions to the IIS and everything is fine with the site now.

The website runs with the permissions of the application pool identity user which is not the same as the ftp user which we opened through plesk. It is a procedure I've done many times (open ftp user-give ftp user to a client), but this time something is just too fishy.