Rsync from Ubuntu server to mounted NAS - Input/output error (5)

I am trying to rsync some backup files from a ubuntu server into a mounted NAS.

All works well except for a big 13GB gz file which shows this error:

rsync: close failed on "/mnt/backupnas/chbsl018/.chbslds018-OALit_db.gz.6L8sHs": Input/output error (5)

rsync error: error in file IO (code 11) at receiver.c(856) [receiver=3.1.0]

The original 13 GB file is called chbslds018-OALit_db.gz and is a gzipped sql file.

The rsync command called is:

rsync -avz -c -h -H --delete --stats /data/backups/backup_db/ /mnt/backupnas/backup_db/

Any idea why I am getting this error?

Update:

In the syslog, I can find this:

Nov 17 17:16:11 backup kernel: [4089920.125948] CIFS VFS: Server 172.16.0.16: has not responded in 120 seconds. Reconnecting...
Nov 17 17:17:01 backup CRON[13015]: (root) CMD (   cd / && run-parts --report /etc/cron.hourly)
Nov 17 17:18:35 backup kernel: [4090064.643918] CIFS VFS: Server 172.16.0.16: has not responded in 120 seconds. Reconnecting...
Nov 17 17:23:40 backup kernel: [4090369.179512] CIFS VFS: SMB response too short (0 bytes)
Nov 17 17:28:21 backup kernel: [4090650.480562] CIFS VFS: Server 172.16.0.16: has not responded in 120 seconds. Reconnecting...