Ah, you are with Strato.
You need to go into the "expert settings" section, and reduce the "split" setting down to 20Mb. Then start a new backup.
Strato have configured their webservers to disallow the creation of any files bigger than 20Mb. So, when UpdraftPlus tries to create a zip containing your files, it gets forbidden to go further once the file gets up to 20Mb.
David