Something happened when I tried to add a Zigbee device, not sure what but Hubitat became unresponsive. Would seem to boot up but never go to the next screen. I finally got into the diagnostic tool by repeatedly hitting refresh on my browser and got in before it recycled again. Tried Safe Mode, tried Soft reset, tried to load a backup from my desktop; all still had the Hubitat rebooting ad nauseam.
Finally I tried the Advanced>Hard reset. It said it reset, then said it would shut down. After a few minutes, I went and restarted it by unplugging re-plugging and tried to find it via 'Find Hubs'. The normal discovery could not find it at all. If I tried 'Advanced> Search by MAC' it found it right away. But, when I try to access it in any way I get nothing; no startup page, no diagnostic tool, etc.
The light in front now just cycles off (10s) to Blue (20s) continuously.
FWIW, I also tried the reset button on the bottom.
Prior to this it had been working flawlessly for about 9 months, controlling a light via sensors and drivers/rule machine; and a valve via another set of sensors and rules.
It was just blinking blue. Since it happened when I was in the middle of pairing and I was actively negotiating with the hub I never thought once about checking the power supply. Interestingly, it took 3 different power sources (all theoretically rated at 2.4A) before one worked; maybe it was just the repeated on/off; who knows.
It works now and my backup restored everything but the pairing but I only have 2 devices so no biggie.
Thanks for the help and not being the guy who just loves to point out deficiencies without actually helping.
I have 2 bricked C7s, after the update to latest firmware. Were running fine for 3 years, same power supply, plugged into ups, living on my network. 3rd Hub, seems fine so far on the same latest firmware. No real solution. I tried all similar actions as you did. Power, ping hub, wait to see it respond, then try and load Gui , Can't connect, got Diagnostics tool to work, but options there seemed to do little. Did a soft reset, and loaded a cloud backup, but it would crash again and again. I woke up to a lot of lights on this morning that I have to go turn off.
Step 1. Press and hold the reset button on the bottom of the hub for 7 or 8 seconds with a toothpick or paperclip and release. Wait till hub light turns blue. This only resets the the network connection, not the hub itself.
Download and install Advanced Port Scanner from https://www.advanced-port-scanner.com/ and scan your local network. (Note the MAC on the bottom of the hub) and compare it to the new ip you have.
They all have static IP addresses. I never lose that. I can see them easily in the Ubiquiti dashboard. They lose the ping. They lose the gui. At no point do they change IP addresses. They just stop responding. Doesn't matter which PC, etc. I can repeat this process over again
I get that, but I'm giving you a diagnostic path. I would also grab a switch then and plug laptop, and hub into it and see if you can get to it that way. (isolated off main network)
They lose the ping like clockwork from the network. Almost to the second , from the time they start responding. Regardless of the PC. The second one is in a remote building. I use Ubiquiti's POE Adapters to get 5 volts at 2 amps for each controller so I can reboot them remotely. I don't feel it's a network issue. But do have two of 3 hubs now doing it. I have several in the field at customers locations and I am not going to update them yet.
If possible, use a fast ethernet switch. The network interface on Hubitat C-7 and C-5 hubs will crash if it encounters a jumbo ethernet frame. I suspect this is the reason why your C-7 hubs crash unexpectedly. Jumbo packets are not possible with a FE switch.
Also, be aware that not all PoE adapters work well with the z-wave radio on C-7, C-8, and C-8 Pro hubs. Please see this detailed post from @dennypage
These are the adapters that I use on my jobs and home, for 5 volt stuff. I can reboot them remotely by power cycling the Ubiquiti switch port. Instant PoE to USB Converter - Ubiquiti Store
My switches are home are USW Flex 2.5G 8 PoE in most places. I tested the C7 on that hub and a laptop, same crash about 3 minutes in. Using Power supply that just came in the C8 Pro box for the hell of it. After 3 minutes, the ping continues to work. Gui dies off.
If I access the Gui a few times, then It causes the Ping to die off too.
A reboot brings it back, and repeats the same process over and over again.
My C8 Pro just came, so I am going to get that going for now, and see what can be done with the C7, so I have a spare.
I wish you luck in figuring out the issue, as you have apparently determined that the suggestions provided in this thread are not relevant to your particular situation.
So, you are using two UniFi Switch ports per hub? One for network connectivity and one for PoE to USB power? The "Instant PoE to USB Converter" device does not appear to be a Poe Splitter, but rather just a 5VDC 2A USB power source, correct?
How are you powering the USW Flex 2.5G 8 PoE switches?
With another upstream PoE switch?
PoE+ or PoE++ or PoE+++?
Or with an external power supply?
The Ubiquiti 210W power supply?
Do you have any VLANs configured? Are the HE Hubs and the computers you're using to access them on the same VLAN? Any chance you have some advanced network intrusion software enabled somewhere that is somehow shutting down communications between your computer and the HE Hubs?
And one final question which was hinted at above, but I did not see you respond...
What I have has been in place for 3.5 years, since moving in. No changes, other than to upgrade switches over time. Shoemakers shoes...
No jumbo frames. 1 Vlan for the cameras. 2 ports per Hubitat on the power adapter, they are older at this point. . One for the adapter, and one for network..
In one location, it's all Solar, with 12 volt POE switches, (non ubiquiti) and a 12 volt to 5 volt converter for the Hubitat. That hubitat works fine, and is current. 1 out of 3.
I ran single mode Fiber between switch locations in some places since they are on the other side of my property. I used to do it with Ubiquiti PTMP bridges.
I'm using the Ubiquiti 210 watt supply in 2 locations with Hubitats. The one switch powers cameras, Access point and IP phone. They are fiber fed between them, so no downstream POE coming in. I have also tried the cube that came with the C8, all the same, 5v, 2amp. I get the same results.
I've had the main hubitat sitting on my desk, so I can see the lights, and reboot it manually, same switch as the PC and laptop. It will lose connection after 3 minutes. When it does boot, into the Gui, the device page just spins and spins, till the ping vanishes. Then it's offline again the light stays green on the hub.
Using an OEM power supply and using a switch, isolate the hubitat to eliminate or confirm the issue is with the hubitat....Plug laptop directly into the switch . Using advanced port scanner see if you can see it. Should be ok if you are using static ip's
Again, if that doesn't work, do the network reset on each hub using the button on the bottom.
the most important 
