Just to be sure it’s not about disk space:
df -h /home
If the sync is interrupted, you could try to increase the input/execution time by adding the following to zzz_nethserver.conf:
php_value[max_input_time] = 3600
php_value[max_execution_time] = 3600
Maybe it helps to customize the nginx config by adding the following to nginx.conf
fastcgi_read_timeout 300s;
You can find more information in the Nextcloud docs.
Do you run another reverse proxy in front of the NS8? Maybe you need to configure the timeouts there…