Where is SCC hosted , is it in customer network or your own network ?
If all the devices are showing the same error then it could be related to network issue. One possibility is that if there is a separate management link and it is down then changing heartbeat will not cause any change. Get the network connectivity checked if all the devices are showing " Disconnected: network time out"
Hi, I've seen this after a device has been offline for several days eg: due to WAN link issue.
Unless you've already tried it and providing the Steelhead can ping the SCC;
From the offending Steelhead CLI :
no cmc enable
Then a minute or two later show scc / show cmc (depending on RIOS version)