Hi Axel, thank you for testing NS8 Alpha on Debian 11! 
Any error message in the Log page?
As NS8 is still in alpha, I’d suggest to reinstall. The quickest way to do it without reinstalling the OS (because you installed on bare metal) is to run the uninstall script, which destroys all your data 
bash /var/lib/nethserver/node/uninstall.sh
Be careful, you’ll find many rm -rf commands in it: check if it’s fine for you.
An alternative soft approach is to remove only the Nextcloud module from the Software page, then install it again.
As we’re talking about bare metal, I’d mount the /home directory on a separate disk/partition. Anything that eases the filesystem growth, like LVM (?)… Debian sysadmins, please help me here 