Thanks to this discussion https://wordpress.org/support/topic/my-backup-is-stuck/ I found a solution to triggered updraftplus which remained very quite in the last weeks.
I actually switched for performance reasons from wordpress internal cron to an external cron. I did so 3 weeks ago. WordPress and all the many plugins were working flawlessly but UPDRAFTPLUS.
Actually, I can confirm Updraftplus doesn’t work with an externalised cron process. But I can give you a manual solution if it gets stuck: just run manually the external cron as soon as you see it got stuck. In other words, run
https://example.com/wp-cron.php?doing_cron_job
and it will trigger the process.
Guys at Updraftplus, I seriously think you should pay attention to your process, it doesn’t make sense that an external cron breaks your plugin. Thanks.