Is it restricted in any way? Or the size (upload size in editor is different from site assets)? Or the extension?

On upgraded site there's sometimes an issue with the folder rights.
I would first try to save the rights on the root folder (just saving should be enough) and copy them to the child folders (if you currently you don' have any specific rights set on one of the child folders).

Which HTML editor is your site using?  You should be using the DNNConnect.CKE editor.  

If you are using the old editor that includes Telerik components, I think that's way the upload is failing.

I'm getting the following error logged when I go to Open HTML Editor Manager which maybe the root of my problem. I did check folder rights and resynced folder and sub folders.

AbsoluteURL:/Default.aspx
DefaultDataProvider:DotNetNuke.Data.SqlDataProvider, DotNetNuke
ExceptionGUID:c53c110a-f2cf-47d5-a8a0-8aea95b96d3f
AssemblyVersion:9.3.2
PortalId:0
UserId:1
TabId:92
RawUrl:/Host/HTMLEditorManager?portalid=0&portpopUp=true
Referrer:http://www.aaaaa.com/Desk...r/index.html?cdv=305
UserAgent:Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/76.0.3809.132 Safari/537.36
ExceptionHash:ANOBH1mDQemWH34kpp2Y1DZX6aI=
Message:Method not found: 'System.Collections.ArrayList DotNetNuke.Common.Utilities.FileSystemUtils.GetFoldersByUser(Int32, Boolean, Boolean, System.String)'.
StackTrace:
InnerMessage:Method not found: 'System.Collections.ArrayList otNetNuke.Common.Utilities.FileSystemUtils.GetFoldersByUser(Int32, Boolean, Boolean, System.String)'.
InnerStackTrace:at DotNetNuke.Providers.RadEditorProvider.ProviderConfig.BindConfigForm() at System.Web.UI.Control.InitRecursive(Control namingContainer) at System.Web.UI.Control.AddedControl(Control control, Int32 index) at DotNetNuke.Modules.HtmlEditorManager.Presenters.ProviderConfigurationPresenter.View_Initialize(Object sender, EventArgs e) at System.EventHandler.Invoke(Object sender, EventArgs e) at DotNetNuke.Web.Mvp.ModuleViewBase.PageInitComplete(Object sender, EventArgs e) at System.EventHandler.Invoke(Object sender, EventArgs e) at System.Web.UI.Page.OnInitComplete(EventArgs e) at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
Source:
FileName:
FileLineNumber:0
FileColumnNumber:0
Method:

AbsoluteURL:/Default.aspx
DefaultDataProvider:DotNetNuke.Data.SqlDataProvider, DotNetNuke
ExceptionGUID:d8ed42fa-acbe-4335-a0dd-bf78a9d0d62c
AssemblyVersion:
PortalId:-1
UserId:-1
TabId:-1
RawUrl:
Referrer:
UserAgent:
ExceptionHash:tNw7TADMRJO4tgmZlk6NU4PNPxM=
Message:The following resource file caused an error while reading:
StackTrace:at DotNetNuke.Services.Localization.LocalizationProvider.GetResourceFileCallBack(CacheItemArgs cacheItemArgs) at DotNetNuke.Common.Utilities.DataCache.GetCachedDataFromDictionary(CacheItemArgs cacheItemArgs, CacheItemExpiredCallback cacheItemExpired)
InnerMessage:The relative virtual path providers/htmleditorproviders/ckeditor/module/app_localresources/editorconfigmanager.ascx.portal-0.resx' is not allowed here.InnerStackTrace:at System.Web.VirtualPath.FailIfRelativePath() at System.Web.Hosting.HostingEnvironment.MapPathActual(VirtualPath virtualPath, Boolean permitNull) at System.Web.Hosting.HostingEnvironment.MapPath(VirtualPath virtualPath) at DotNetNuke.Services.Localization.LocalizationProvider.GetResourceFileCallBack(CacheItemArgs cacheItemArgs)
Source:DotNetNuke
FileName:
FileLineNumber:0
FileColumnNumber:0
Method:

I found this post that seems to have fixed my issue but still testing.

https://www.chrishammond....-HTML-Editor-Manager