Your site is the ultimate stress-test for a backup plugin, due to the large number of huge files.
I've put in some improvements to UpdraftPlus, but for now have hit a brick wall. You have MP3 files of 300Mb each, but your PHP memory limit is only 128Mb. The PHP function that adds files into zip files fails - I presume because it wants to read some, if not all, of each file into memory. I expect that it's probably not possible to add a file to a zip which, when compressed, is bigger than memory (and MP3s are already compressed, so it'll still be 300Mb after compression).
I'm going to add a new method for handling zip files to UpdraftPlus that can potentially break outside of PHP and its memory limits. I'll come back to you then.