The software technically does not come from your server, it comes from your SDP (which may be the same machine) which is an "agent" thing. So restarting the SDP's agent would be more effective here than restarting the server.
Observe a specific package. Does it fail for all machines? Or just some? That will help indicate if it's a problem with he payload itself (in which case, re-upload that payload), or if it's a problem reaching the SDP.
Another thing you can do is enable debug on the SDP agent, and check logs there.
We operate a single LANrev server so the SDP was restarted with the restart of the LANrev server.
A specific package has failed for some agents and succeeded for others.
A specific agent has been able to install some packages successfully, where it fails the download on other packages.
I will enable debug logging and see where that takes me.