It might be because ataimeout has activated.
Set
OB2MEDIUM_KEEP_ALIVE_INTERVAL=1800
in the Gateway/MediaAgent server omnirc file.
With this setting Keepalive packets are sent more often.
You need DP 9.03 or 9.04 for that value to be affective.
Also make sure the StoreOnce Appliance firmware is up to date.
3.1.3 has an adjustment that can help prevent this error.