Are you sure that the version is 2.3.9.6 and not the 2.3.9.7 because I have a similar issue you can see the post in
https://www.mojoportal.com/Forums/Thread.aspx?pageid=5&t=11690~1#post48661
The mojo version 2.3.9.7 have a new jquery file uploader so the post about config the max file size for netupload it's not valid anymore, and the error that you post sound to me like a missing config in the web.config.
Because neatupload it's not longer supported the web.config secction for the configuration of neatupload was deleted in the last version of mojo
You can read the release note here
https://www.mojoportal.com/mojoportal-2397-released