这两个位置（通常）在同一个磁盘上，所以不可能是磁盘空间问题，因此我确实认为一定是权限问题。当然，备份通常会正常工作——假设您进行了标准安装——所以您手动创建目录似乎与问题有关。
但是。我在我的机器上检查了一下，在容器内部，所有权和保护是这样的：
root@ubuntu-2gb-nbg1-1-app:/var/www/discourse# ls -ltrd /var/www/discourse/tmp/backups/default /var/www/discourse/public/backups/default
drwxr-xr-x 2 discourse www-data 4096 Jan 24 03:36 /var/www/discourse/tmp/backups/default
drwxr-xr-x 2 discourse www-data 4096 Jan 24 03:36 /var/www/discourse/public/backups/default
root@ubuntu-2gb-nbg1-1-app:/var/www/discourse# ls -nltrd /var/www/discourse/tmp/backups/default /var/www/discourse/public/backups/default
drwxr-xr-x 2 1000 33 4096 Jan 24 03:36 /var/www/discourse/tmp/backups/default
drwxr-xr-x 2 1000 33 4096 Jan 24 03:36 /var/www/discourse/public/backups/default
root@ubuntu-2gb-nbg1-1-app:/var/www/discourse# ls -nltrd /var/www/discourse/tmp/backups /var/www/discourse/public/backups
lrwxrwxrwx 1 0 0 19 Jan 10 15:26 /var/www/discourse/tmp/backups -> /shared/tmp/backups
lrwxrwxrwx 1 0 0 15 Jan 10 15:26 /var/www/discourse/public/backups -> /shared/backups
root@ubuntu-2gb-nbg1-1-app:/var/www/discourse# ls -nltrd /var/www/discourse/tmp /var/www/discourse/public
drwxr-xr-x 1 1000 1000 12288 Jan 10 15:31 /var/www/discourse/public
drwxr-xr-x 1 1000 33 4096 Jan 10 15:36 /var/www/discourse/tmp
root@ubuntu-2gb-nbg1-1-app:/var/www/discourse# ls -nltrd /var/www/discourse
drwxr-xr-x 1 1000 1000 4096 Jan 10 15:26 /var/www/discourse
root@ubuntu-2gb-nbg1-1-app:/var/www/discourse# ls -nltrd /shared/backups /shared/tmp/backups
drwxr-xr-x 3 1000 33 4096 Aug 15 2019 /shared/backups
drwxr-xr-x 3 1000 33 4096 Jan 10 15:36 /shared/tmp/backups
root@ubuntu-2gb-nbg1-1-app:/var/www/discourse# ls -nltrd /shared /shared/tmp
drwxr-xr-x 13 0 0 4096 Jan 27 2021 /shared
drwxr-xr-x 4 1000 33 4096 Jan 10 15:35 /shared/tmp
1000 用户是“discourse”，33 组是“www-data”，这一切都不能让我明白您的问题所在。抱歉。