I am afraid that’s a bit beyond the scope of the forum. Plus, this really might be a violation of the terms of service.
I am afraid you will have to debug this with the developer of that client. Once you know what the error is, you can probably adjust some settings.
Typically you might run into a 524, the upload limit of 100 megabyte, or some protocol details. And are you sure that’s HTTP? If not, that cannot work.