The channel this is referring to is between the web client and the Laserfiche server. There may be more details in the event logs of the web client machine. Often we see this being caused by some firewall or other security product closing the connection due to either the size of the response or the content type (e.g. pdf). Experiment with temporarily disabling anything like that, or adding exclusions for the web client-lfs route. The next step would be to open a support case.