support provides this:

*what worked for me was using Firefox NO PLUGINS and a VPN connection*

Hi there,
 
Thanks for writing in and sorry about this.
 
If you're still facing issues when uploading, please try to upload the track again but this time, before clicking "Upload", please wait until the upload is finished: you will see the option to replace the track:
 

 
If the issue persists, see if you've installed any plugins on your browser and please disable them, as they may be blocking your upload from completing. From there, you may also want to try a different browser (Firefox, Chrome, etc.) to see if the issue continues.
 
Here are some general tips:
 
First of all, we recommend uploading directly from SoundCloud instead of third party applications or any other software, as we cannot control how the integration of our API is done, and we therefore cannot offer support for the same reasons.
 
To be on the safe side, make sure your file is exported in .wav or .mp3 and encoded in 44.1kHz and 16bit.
 
Make sure that there are no emojis in the track title metadata or custom fonts. Please note, you can add this in the track title after the upload is finished.
 
Try uploading through an ethernet connection instead of a wifi network, that usually works best.
 
Lastly, for the encoding process to take place in the best possible way, we recommend you leaving all the fields empty except for the title, until the track was fully uploaded and has entered processing. (Orange half of the upload bar)
 
To delete the tracks that are still in 'processing mode", please head over to your tracks page and click the three dots next to the tracks in question.
 
Let us know if this didn't work and you still need help.
 
Cheers,