Bootloader bricked after botched upgrade over PoE?

[Borromini]

I messed up while sysupgrading an EAP615-Wall, bumping into the cable from the PoE switch (which did not seem to be fully plugged in, I know... ). It looks like the upgrade botched, and OpenWrt isn't booting anymore.

A good Samaritan soldered me a serial header, so I could hook it up and TFTP an image to the bootloader, then sysupgrade. So far, so good. The AP didn't come alive at the first try, but I thought it might be my PoE switch. However, while the TFTP boot and subsequent sysupgrade went fine, when rebooting the device didn't come up. The switch doesn't seem to get a request to deliver power, and it takes quite a few tries before it does and the AP shows signs of life...

Despite being able to TFTP boot first, I am now only seeing this:

U-Boot SPL 2018.09 (Feb 20 2021 - 09:43:17 +0800)
Trying to boot from NOR
Error: LZMA uncompression error: 1
Trying to boot from UART

Failed to load U-Boot image!
Entering emergency mode.
Please transmit a valid U-Boot image through this serial console.
The U-Boot image will be booted up directly, and not be written to flash.
Accepted mode is Ymoden-1K.

I didn't see this the first time the device came up, which suggests the bootloader got corrupted after somehow? Is that possible? Darn tiny window if so .

I haven't tried to feed it a U-Boot image yet. I'd like to know if this suggests a hardware failure though? If I get this device going again it will be deployed remotely, I cannot just get to it to replace it...

Thankful for any insights.

[Borromini]

Alright, so I uploaded a U-boot backup through minicom. That makes the device boot and I get the OpenWr prompt, I then wrote the u-boot backup to /dev/mtd0 thinking it might have been corrupted. However, when I tell OpenWrt to reboot, the same thing happens all over again: it fails to load U-boot...Any suggestions?

Thanks.

===================================================================
                MT7621   stage1 code done 
                CPU=500000000 HZ BUS=166666666 HZ
===================================================================

U-Boot SPL 2018.09 (Feb 20 2021 - 09:43:17 +0800)
Trying to boot from NOR
Error: LZMA uncompression error: 1
Trying to boot from UART

Failed to load U-Boot image!
Entering emergency mode.
Please transmit a valid U-Boot image through this serial console.
The U-Boot image will be booted up directly, and not be written to flash.

[Borromini]

I kept banging my head at dd writing back a fully functional U-boot backup that would yield a different (but consistent) checksum when read back from flash until PaulFertser on IRC suggested I try the mtd utility. After erasing /dev/mtd0, then writing the backup to it, both with mtd, both checksums matched, and the AP boots through again like before.