This has been going on with my old Buffalo WZR-600DHP for a while now:
- I build a new image (typically once a new release is available)
sysupgrade-flash the new image, keeping config; "succeeds" based on its output
- Router boots: Red LED blinks the "periodic two blinks of flash corruption".
- Power off, wait just a few minutes, power on: same problem.
- Power off, wait for a while (up to a few days)
- Power on the router: It now boots into the new image, config correctly preserved.
Lately it's also started doing this without a flash: Just a power cycle will put it in the bad, "requires rest" state.
I'm theorizing that some capacitor has become "tired", or for some reason like degraded solder it fails to drain. That's the only thing I can think of that matches the profile of "wait a few days then try again".
Thought I'd run this by the community in case there's a reasonably easy fix. It's been decades since I last used a soldering iron, so that gives you an idea of my hardware skills. Still, if at all possible, I'd like to save this box from becoming e-waste.
Edit: The above has been happening starting with the 17.x series and through 18.06.5.