What boggles me is that I DIDN'T change anything in the rendering options. I used the exact same option as another video I made and that one has no problem.

Here are the settings used to render:

MPEG-4
Scanning auto Bitrate
encoder threads 1
video 12000
audio 128
full project
Render to file X.mp4

I can't view it in Ubuntu or on Windows. But another .mp4 using the same settings is viewable! What's going on?

When I try to play the video I get an error saying "This file contains no playable streams".

Strange, I can watch the video if I render it to .mpg or .avi but not if I make it a .mp4 edit: the framerate of the .mpg is horrible though. It's like it's playing at 5 frames per second.

And now the .avi gives me: "Internal Data Stream error"

Between the last working rendering and this one: Were there updates to mlt or ffmpeg? I have the feeling that kdenlive is the wrong place to dig. A long time ago I have had some trouble when mlt had the wrong version (either too old or too new for kdenlive).

You could try to render a part of your vid to lossless and then use ffmpeg to transcode to mp4. If that works, you at least have a workaround to get your video out.

Well, now that you mention it, it's possible that I might have downloaded/installed some general updates while I was working on that project. Though I'm unsure if that's the cause of the problem.

Anyway I have found that .mpg works fine. The framerate wasn't bad the video was just playing back really fast and skipping parts so it appeared like a slideshow. However the file opened and played fine in a windows computer.

The .mp4 and .avi still have errors. But at least now I can upload it.

If the errors are due to Ubuntu updates maybe all I need to do is restart the computer?

It's quite unlikeley but - if in doubt: reboot.

Watching the complains about rendering problems and stalls, especially ubuntu seems to be prone to such problems. Other distros (e.g. openSuse, Archlinux) seem to be more stable and are rarely the source of complains. Maybe using the sunab repsository for kdenlive might help you.