Alright so I was trying to restore some files from my backup tapes for a user and the restore job kept failing again and again with the same message saying
"E8523 - Failed to send data to agent - CA ARCserve 12.5"
So I changed the values for the registry keys found under
HKLM\SOFTWARE\ComputerAssociates\CA ARCServe Backup\ClientAgent\Parameters
ReceiveTimeOut and SendTimeOut to a decimal value of 3600 from 1200.. some suggested changing it to a higher value in case the job failed but I stuck to the former and changed it to 3600 and then stopped all my CaArCserve services by running to batch file "cstop.bat" which can be found under "C:\Program Files (x86)\CA\ARCserve Backup" and then started the services by using another batch file "cstart.bat" found under the same location.
After that I started the restore again and this time it worked! So I guess the registry change did indeed work and I did not even have to reboot the server!