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On Ubuntu/Debian Linux you can verify the disk is full with
df -h
Then you'll either need to increase the disk size or delete some things in order to get space back
Common places to check for UniFi storage issues:
Backups directory
Syslog directory
To delete all backups:
cd /usr/lib/unifi/data/backup/
rm *.unf
cd /usr/lib/unifi/data/backup/autobackup
rm *.unf
To delete all syslogs:
cd /var/log/unifi/remote
rm *.log
After your done, you might want to change your backup settings to backup less frequently or a reduce the maximum number of files
You can manage your backup schedule under Settings > System > Backups > Reschedule
This is a server wide setting
You should also disable any Internally Stored Activity Logging
This can be done under Settings > System > Integration
This is a site specific setting so it needs to be done for each site on your server