Voice Universe - Can't upload any files to voice universe

I've already created the voice I want to train, but I can't add ANY files to the drag-n-drop area in the pop up to populate the 15min minimum requirement. It keeps bombing out with a server 0 error.

Impossible to be my internet connection as I'm running 100Mb fibre line and everything is super snappy connection-wise. The site just simply refuses any files I upload, whether they be 10Mb wav files or 100Mb wav files. It doesn't matter, it just keeps failing every single time.