Problem fixed (for now)!!!
Some background on my setup: ASUS RT-N66U router, Mediacom Triple Play package with internet + whole home Tivo (that uses an Actiontec ECB2500C MoCA adapter) + phone (that uses a separate Cisco DPC3208 VoIP modem).
SHORT STORY:
- Turn off every device on your LAN except for the S5. Every single one, including your modem, router, cable boxes/DVRs, MoCA adapters, VoIP gateways, etc.
- Turn everything back on, starting with the MoCA adapter, then your modem, then the switches the VoIP gateway is connected to, then everything else.
- Every time you turn something on, check the S5 to make sure everything's working. If the problem returns at some step, then the most recently added device might be the culprit.
- Be sure your VoIP gateway is not connected to your LAN via Ethernet. I think that causes something(s) on the S5 to see it as an internet gateway, except it doesn't really have internet access, which leads to problems.
The S5 should load images normally now.
LONG STORY:
- I took the advice on the thread (thanks!) and created a guest network on my router. When that fixed the issue, I figured my router was working correctly and so the problem had something else on my LAN, since the guest network is separate from the LAN.
- Given the above, I started disconnecting devices from the LAN and checking the S5 after each one.
- Upon disconnecting the VoIP modem from the LAN, I saw a slight improvement.
- Upon disconnecting the MoCA adapter, the problem went away entirely.
As I said, I think something (maybe a partial power loss in the LAN as my modem and router are connected to UPS but much of the rest of the LAN isn't) made either the modem or the MoCA adapter appear as a gateway to some S5 apps/functions. Powering everything down and then back up in the order I specified forces your router to rebuild its routing table for the LAN from scratch as devices come online (as opposed to seeing them already there after a reset). I'm hoping my solution lasts, but if it messes up again I'll probably just power down the MoCA adapter, my modem, and my router only and then turn them back on in that order.
Hope this helps folks.