Do the machines at this remote site have gateway certificates for your gateway?
I dont believe that your clients at this site would be able to use the gateway for software distribution if they cant see the core unless they have a management gateway certificate installed.
If you were to look in the C:\Program Files\LANDesk\Shared Files\cbaroot\broker folder on one of these machines, you should see a broker.crt .csr and .key
Of course this may already be the case since you are in a management gateway environment already. But it seemed sensible to ask looking at the error.
On another note, within BrokerConfig.exe located in C:\Program Files\LANDesk\LDClient on the remote site PC, under the Gateway Information tab, is the IP address assigned for the management gateway correct?
The machines definitely have the correct gateway config; they can (and do) still send inventory scans, etc. '
Now when the policy is being collected what does the proxyhost.log file say on the client?
Also does the taskmanifest.ini mentioned in the logfile you posted appear in ldlogon\filelists\ ?? (If not either restart the task or create a new one)
Is there a proxy running at the remote site?
Have you tried creating the package again from scratch?
Sorry to ask so many questions, but these ones can be tough cookies to crack!