[FS#840] AR71xx - Recent firmware won't flash (building without errors)

[LEDE Bugs]

The following task has a new comment added:

FS#840 - AR71xx - Recent firmware won't flash (building without errors)
User who did this - juanriccio (juanriccio)

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Tried upgrading again to r4610.

No way, it won't actually upgrade. This has started when I got r4335 in: it was a same-day build.

What's funny is that I have three TP-Link TL-WR841N/ND v8 routers (used as simple access points), and all three showed and show the exact same behavior. All of them upgraded nicely just up to, but not beyond, r4355.

I only tried upgrading via LuCi. How can I diagnose the situation and get some log or other information without bricking them?
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More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=840#comment3054