That could be because of a server name change, and because the old server is not present on the server table. You need to manually “setup” your agents in order to use the new server. To avoid editing them one by one, you can use the SQL manager to configure all the agents with a quick SQL command.
For example, if your new server is called linux-ppby the SQL sentence will be:
update tagente set server_name = "linux-ppby"
You won’t need to restart the server, just wait a few seconds and all modules will be processed again.
Another choice is to change the “Server” parameter in the ‘massive agent edition’ option of the Massive Operation section, by selecting all migrated agents.