Ive had this error reported to me a few times by different users but never been able to sort it. It happens when adding attachments, this user has reported it happens with firefox but not with chrome using the same file. he has also reported that if he disables flash in firefox he can successfully upload the file.
I have tried Flash Uploader disabled/enabled it doesn't make a difference still get error.
he is using firefox 43.0.4 the same as me but im having no problems.
If this issue is sporadic, it's unlikely to be a server issue. It sounds like it's more on the client side. We have seen some examples with the Flash uploader being a little less resilient to network hiccups than a standard browser connection, but unfortunately we have no ability to control or influence that. That fits with it working when he doesn't use the Flash uploader, for example.