@Dennis: thanks for posting the work-around.
At the moment I am pretty lost what else to try. I have 3 test OS on my own network, one OS 2.3 DC, two OS 3.2 AC+Ethenret, two are directly connected to my router and one is corrected through a powerline Ethernet adapter. All three are running 2.1.9(5) and have been alive for more than 5 days since I flahsed 2.1.9(5) onto them. I run the Test script (http://raysfiles.com/os/TestOSManual.html) on all three of them, with a browser tab open on the side to show homepage status, and have IFTTT set up to receive notification on station runs. I have encountered a couple of cases where IFTTT notification was missing (debugging information shows it wasn’t able to connect to IFTTT server at that moment). But otherwise the 3 controllers have been running fine, no hanging, no issue accessing them.
So the issue some of the other users are experiencing is pretty much beyond my knowledge — the only way to find out would be to go to their home in person to debug the issue… The feature implemented in 2.1.9(6) debug version — performing a Ethernet reset when the microcontroller detects ENC28J60 registers are in one of the erroneous states, is akin to performing a reboot, though it’s a softer reboot than a power-on reboot. But it seems even with this, it doesn’t solve the problem for Water_my_lawn, and some of the register values reported, like ECON1=80 or 84, I have’ never seen these values on my controllers. In any case, I am pretty lost and I have to move on for now to other priorities and come back to it when anyone has more insight to what’s happening.
Regarding work-around, in addition to what Dennis mentioned (using a timer to trigger a power-on reboot once a day), I still think using a secondary router is an effective approach — there are inexpensive routers less than $20, it won’t affect access to the controller from the primary network as long as you set up port forwarding on the secondary router.
Another work-around, for OS 3.2 users, is to try W5500 Ethernet module (https://opensprinkler.com/forums/topic/instructions-for-testing-os-3-2-with-w5500-ethernet-module/). Though, this only works for OS 3.2 and doesn’t work for OS 2.3.