"Interesting".
From the screenshot it looks like the the attempt to open the file to write to on line 1763 of updraftplus.php works (because the file is there):
$this->backup_db_open($updraft_dir.'/'.$table_file_prefix.'.tmp.gz', true);
But it seems it must crash a few lines later when attempting to write to the file, as the file in the screenshot has size zero.
So it looks like the gzwrite() call inside stow() (line 2011 onwards) fails. If you're having other problems with writing to disk on that site, then they're probably related. Account quote full, perhaps? (Though strange it'd happened on the very first write). Or broken PHP install in which gzwrite() is broken.
David