I must admit that I'm unsure if this is an issue with Forum 4.5.3 or with DNN 5.1.2 as I haven't uploaded an attachment in a very long time but was just trying to upload an attachment today on my forum and receive the following error (I've tested it logged in as Host, Admin, and a "regular" user):

InnerException: Object reference not set to an instance of an object.
FileName:
FileLineNumber: 0
FileColumnNumber: 0
Method: DotNetNuke.Security.Permissions.PermissionProvider.CanAdminFolder
StackTrace:
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.Security.Permissions.PermissionProvider.CanAdminFolder(FolderInfo folder) at DotNetNuke.Common.Utilities.FileSystemUtils.UploadFile(String RootPath, HttpPostedFile objHtmlInputFile, String NewFileName, Boolean Unzip) at DotNetNuke.Common.Utilities.FileSystemUtils.UploadFile(String RootPath, HttpPostedFile objHtmlInputFile, Boolean Unzip) at DotNetNuke.Modules.Forum.WebControls.AttachmentControl.cmdUpload_Click(Object sender, EventArgs e) --- End of inner exception stack trace ---

I can work around it by uploading the doc using the file manager and then linking it to my forum post but that isn't exactly a "fun" workaround.  Any ideas?

Nope, the file I was testing with was a mere 2kb after the initial failure (which was around 90kb).  The failure above was not a "Request Exceeded Maximum Length" error which you would expect when the file is larger than 4mb (standard ASP.NET limitation).  As it stands, and as you mentioned DNN is set up with an 8MB limitation instead.  At any rate that is definitely *not* the problem.

Are you serious?  All of that is in my original post -- I tried as Host, Admin, and a regular user with upload rights.  It is an attachment so you don't get to choose the path, you simply upload it by attaching it to the thread.  (i'm not talking about using the filemanager link in FCK editor, I'm talking about the actual ATTACHMENT functionality in the Forum Module).

Maybe I'll just download the source and figures it out.