Same issue here, just de-activate wp-stateless plugin, make the backup with the same data of UpdraftPlus WordPress Backup Plugin (it worked now) and re-activate the wp-stateless plugin.
It is a shame that stateless people don’t fix the issue as it is likely that if you want to use google cloud storage for wordpress it is logical that you will use it for both, images and backup, so they are likely to collide.
