I'm almost certain this is only happening to me as it feels like my extensions are to blame for it.
Running Firefox beta 56 64bit but this has been happening off and on for a couple months now since around FF54 I think. Usually takes turns with the Code 0 error. This problem goes away when I disable all my extensions. I've already tried testing them one by one but none of them cause a problem by themselves leading me to believe this is a combination of more than one faulty extension and possibly the site or Firefox itself. I'd love it if I could go back to uploading without having to restart FF in safe mode all the time. This is why I finally decided to post this and see if you guys can help me figure it out.
Here are the extensions I'm using:
As far as I can tell, only uBlock would have an affect on the site. Tampermonkey is running an anti-anti-adblock script for uBlock. I've tried whitelisting the site and disabling altogether but it didn't make a difference unless I also disable all other extensions. Once disabled, I only run into the Code 0 error sometimes but I'm pretty sure that's because of my slow ass upload speed.
TLDR: This error only occurs when all 16 of my Firefox extensions are enabled; goes away completely when all are disabled and FF is restarted.
I'd like to add that the Clear button on the upload screen fails to remove any of the failed uploads. Clicking it does nothing.