Update existing Instance not working with web Updater

Hello, everyone,
I have four Nextcloud instances running at the same hoster. Unfortunately, one of them cannot be updated.
At the moment it is on 20.0.8 and I want to update to 20.0.10. This is what the updater offers.
It always gets stuck on Step 3 when creating the backup. The backup is also started but not finished.

It always looks like this in the updater log.

2021-05-28T11: 56: 21 + 0200 PdVUt9OJKO [info] currentStep ()
2021-05-28T11: 56: 21 + 0200 PdVUt9OJKO [info] Step 2 is in state “end”.
2021-05-28T11: 56: 21 + 0200 PdVUt9OJKO [info] POST request for step “3”
2021-05-28T11: 56: 21 + 0200 PdVUt9OJKO [info] startStep (“3”)
2021-05-28T11: 56: 21 + 0200 PdVUt9OJKO [info] createBackup ()
2021-05-28T11: 56: 21 + 0200 PdVUt9OJKO [info] backup folder location:
/home/sites/site79/web/nextcloud/data/updater-ocdycnx17ae0/backups/nextcloud-20.0.8.1-1622195781/

The web update says Create Backup Parsing Response failed. If you try to open the text below, there is no further text.
Do you have any ideas what I can do at this point?
Greetings Christian