Recently, we have been getting this error with our CA Arcserve
E8523 : FAILED TO SEND DATA TO CLIENT AGENT
From digging through the web, we found an article from the CA site pointing to this kind of issue.
This talks about modifying the registry and also checking the speed and duplex settings of your NICs, switch, routers, etc…
In our situation, it was network relative. We have to manually configure the speed and duplex of our servers.