NimbostratusClients need to have a route to Citrix application servers when using LTM without APM. My guess is external receiver clients are receiving the application servers internal IP address and it does not have a route to the application servers address. LTM only deployment requires the application server address be publicly accessible.
Enabling APM will allow receiver to application server ICA proxy while enforcing secure connections. APM forces Receiver clients back to the BIG-IP's virtual address and enforces ICA over encrypted web. Once the ICA request is received by APM it proxies the connection back to the originally specified application servers address via standard ICA ports.
