Access to nextcloud denied and the directories - please help!

hello dear community,

unfortunately i can’t get into my freshly set up nextcloud.
i get the error:

Internal Server Error
The server encountered an internal error and was unable to complete your request.
Please contact the server administrator if this error reappears multiple times, please include the technical details below in your report.
More details can be found in the server log.

this all occurred while I was backing up photos and videos from my Android device to Nextcloud via the Nextcloud app.
I selected my external hard drive mounted with SMB as the backup medium.

I installed Nextcloud in a LXC in Proxmox and everything worked fine until this error.
To install Nextcloud I followed these instructions exactly.

I wanted to see the nextcloud.log but unfortunately I can not find it. Access is denied at every point:

root@nextcloud:~# find / -name nextcloud.log
find: ‘/sys/kernel/debug’: Permission denied
find: ‘/sys/fs/pstore’: Permission denied
find: ‘/sys/fs/fuse/connections/42’: Permission denied
find: ‘/sys/fs/fuse/connections/40’: Permission denied
find: ‘/lost+found’: Permission denied
/var/log/nextcloud.log
/var/www/html/nextcloud/data/nextcloud.log
find: ‘/proc/tty/driver’: Permission denied
find: ‘/dev/.lxc/sys/kernel’: Permission denied
find: ‘/dev/.lxc/sys/power’: Permission denied
find: ‘/dev/.lxc/sys/class’: Permission denied
find: ‘/dev/.lxc/sys/devices’: Permission denied
find: ‘/dev/.lxc/sys/dev’: Permission denied
find: ‘/dev/.lxc/sys/hypervisor’: Permission denied
find: ‘/dev/.lxc/sys/fs’: Permission denied
find: ‘/dev/.lxc/sys/bus’: Permission denied
find: ‘/dev/.lxc/sys/firmware’: Permission denied
find: ‘/dev/.lxc/sys/block’: Permission denied
find: ‘/dev/.lxc/sys/module’: Permission denied
find: ‘/dev/.lxc/proc/fs’: Permission denied
find: ‘/dev/.lxc/proc/bus’: Permission denied
find: ‘/dev/.lxc/proc/irq’: Permission denied
find: ‘/dev/.lxc/proc/spl’: Permission denied
find: ‘/dev/.lxc/proc/sys’: Permission denied
find: ‘/dev/.lxc/proc/tty’: Permission denied
find: ‘/dev/.lxc/proc/acpi’: Permission denied
find: ‘/dev/.lxc/proc/scsi’: Permission denied
find: ‘/dev/.lxc/proc/driver’: Permission denied
find: ‘/dev/.lxc/proc/sysvipc’: Permission denied
find: ‘/dev/.lxc/proc/pressure’: Permission denied
find: ‘/dev/.lxc/proc/dynamic_debug’: Permission denied
find: ‘/dev/.lxc/proc/1’: Permission denied
find: ‘/dev/.lxc/proc/43’: Permission denied
find: ‘/dev/.lxc/proc/85’: Permission denied
find: ‘/dev/.lxc/proc/89’: Permission denied
find: ‘/dev/.lxc/proc/100’: Permission denied
find: ‘/dev/.lxc/proc/101’: Permission denied
find: ‘/dev/.lxc/proc/110’: Permission denied
find: ‘/dev/.lxc/proc/111’: Permission denied
find: ‘/dev/.lxc/proc/113’: Permission denied
find: ‘/dev/.lxc/proc/116’: Permission denied
find: ‘/dev/.lxc/proc/128’: Permission denied
find: ‘/dev/.lxc/proc/129’: Permission denied
find: ‘/dev/.lxc/proc/130’: Permission denied
find: ‘/dev/.lxc/proc/132’: Permission denied
find: ‘/dev/.lxc/proc/137’: Permission denied
find: ‘/dev/.lxc/proc/143’: Permission denied
find: ‘/dev/.lxc/proc/189’: Permission denied
find: ‘/dev/.lxc/proc/190’: Permission denied
find: ‘/dev/.lxc/proc/191’: Permission denied
find: ‘/dev/.lxc/proc/192’: Permission denied
find: ‘/dev/.lxc/proc/193’: Permission denied
find: ‘/dev/.lxc/proc/219’: Permission denied
find: ‘/dev/.lxc/proc/375’: Permission denied
find: ‘/dev/.lxc/proc/376’: Permission denied
find: ‘/dev/.lxc/proc/377’: Permission denied
find: ‘/dev/.lxc/proc/433’: Permission denied
/root/nextcloud.log
root@nextcloud:~#

Please help!