**This was originally posted by me on the Wix Experts Forum, but since I got not replies, I'm posting this on this Forum**
I'm like 99% sure this a Wix related bug.
So basically, if you have an uploadButton, set the Supported File Type to Video and write some code to start the upload, it'll return an error. I've tested it in 2 different Sites and the result is similar, though not the same. The similarity is that the error has to do with some mistakes in the uploadButton scripts for when an error occurs. It has to do with JSON. In one of the Sites I found a JSON object inside double quotes which obviously causes problems because the object already contains double quotes.
Please fix this bug :)