sitecore one or more files could not be uploaded

Problem:

Recently I struggeled uploading files into sitecore media libary.

Possible Solutions:

1. Try different Browsers. I switched to IE9, it works.
2. Update Sitecore. Put the latest version on the server.
3. Check Permission on the Media folder. Check the at leas "Sitecore Client Users" shoud have full access. Also file permission of the "Upload" folder in website root could be a problem.
4. Change setting in web.config or settings.config 

<setting name="Upload.Classic" value="true" />

5. Check max file size in web.config
<setting name="Media.MaxSizeInDatabase" value="15MB" />

6. Check time out in web.config
<httpRuntime maxRequestLength=”16384″ executionTimeout=”600″ />