The problem is the files is not uploading at all. I read in your note about jquery
"What are the common problems that can prevent upload to work? In some cases, it can happen that jQuery is not included in the page, even so the bbPress requires it to be loaded. That can happen if something else is unloading jQuery. If the jQuery is not present, upload will not work. Other common issue is that WordPress Media Library upload is not working. If that is not set up, attachments upload can't work."
I have deactivated all of the plugins I use (it's only a few) other than bbpress, of course, but still no upload happens.
Is there another way to check if jquery is running/included?