Firstly - be sure, that you have a free space on the archive server disk(s) and the system has the rights to write/read from this disks.
It is possible to add the server to Administration on client once again? If is not present - to try on IP.
Firewall - ports are set up correctly?
Entry SQL or Windows / Domain?
"We have upgraded to new server" - if is possible more specifically (perhaps something was not so made).
And what in Logs?
So, after switching to the new archive server it was all working ok?
Maybe try restarting both the archive and data servers.