That is curious, but I guess it's not entirely impossible that bootloader v1.0.0.52, while being newer than the known good ones, is incompatible. Known good firmware versions are listed in the wiki. Disproven, see below.
Just to double check, did you follow the installation instructions outlined in the wiki or did you follow any of the instructions spread across this thread? The instructions in the wiki are current best practice.
Any chance I can get a copy of those firmwares?
I received a copy of AP141-6.5r12.img.S firmware but I can't flash it back on. I have the exact same issue you had.
I can write it to the AP using the image_flash command but when I reset it, it always trys to boot the openwrt that doesn't work.
Cheers,
Colin
How often did you try to have it boot? The AP121 can hold two firmwares in flash, and it switches to the primary firmware if the secondary one fails to boot three times. (I don't know if it would switch to the secondary if the primary fails, as that is not an option with OpenWrt taking the space of both.)
Unfortunately there is no known way of manually switching between firmwares from the bootloader console, but on the serial output you should at least be able to observe which one (i.e. which flash address) it is trying to boot.
I just successfully flashed three routers with v1.0.0.52 (also two with v1.0.0.4f), so these should be compatible. The only problem I had was the tftp connection being weirdly spotty (more specifically, the first transfer always had some errors in the beginning, second transfer worked though).
That's great information, thank you, I will add it to the Wiki. v1.0.0.4f is newer than v1.0.0.43, right? Do you perchance know which AeroHive firmware version includes these bootloader versions?
Also, a bit frustratingly, this thread has the weird habit of people abruptly dropping out of the discussion without any feedback.
Great! Yep, that one must be newer, but weirdly even newer than v1.0.0.50? The web interface showed 6.5r4 for v1.0.0.4f and 6.5r8b for v1.0.0.52.
They might have had reasons to go back to a previous version in a newer release, I wouldn't read too much into that. Thanks for the HiveOS version numbers, amended in the Wiki.
Hello,
there can i get the latest HiveOS to update my Bootloader?
Can somebody send it to me?
greets
Hello, I bought one of these access points and just realized none of my computers have a Serial port. Is there any other way I could install openwrt on it wihtout it? thanks in advance
Wiki page says
Installation has to be done through the device's bootloader console accessed via RS232 serial connection, supplemented by a TFTP server connected via ethernet.
so that would be a no.
get a RS232 (RJ45) <> USB serial port, they're < $10.
Pls I need a fullflash of 1M spi flash or the working bootloader of AP121 to boot openwrt firmware because I bricked this stubborn device , thanks
Pls I need a fullflash of 1M spi flash or the working bootloader of AP121 to boot openwrt firmware because I bricked this stubborn device , thanks
Let me see what I can do, luckily I have one with me right now.
Thank you , With your help I will revive a dead device.
(Link expired)
That's the dumps of mtd5..mtd9, i.e. everything on the SPI-connected NOR flash chip. Most importantly the first file (partition@0) contains the u-boot partition on the beginning of the chip at 0x0. I'm not completely sure but I believe the NOR chip doesn't contain the wifi data (that would be wifi-info on the NAND chip), but proceed with the necessary care anyway.
Thank you very much , I download it and will flash the dump tomorrow , thank you for your time and effort 
For others' searches and for possible updating of the device page...
I have installed 23.05.0 successfully on an AP121 that uses bootloader version v1.0.0.33.
Thank you, added to the Wiki.
And few years later ... Hi there, back then I started this thread, have seen the whole thread. Thanks for the debug process. I wonder if you have a stable uboot firmware to successfully install OpenWRT on this devices?
Thanks for your help.
Hey, welcome back.
I believe the best practice has not changed: Install a HiveOS update that brings your bootloader to at least v1.0.0.33, then you have that "stable uboot" that can boot OpenWrt.
As to where to get that HiveOS update, we're still none the wiser: It's still only distributed by AeroHive, or now "Extreme Networks", upon request. In all the years I have still not seen any HiveOS update published anywhere else.