Hi emnoc,
Not sure if the AP is currently booting or not..
When you plug the unit into FortiExplorer and power it on, do you see the prompt to hit a key to enter BIOS menu?
Should be some periods like below, that show up at about 1 per second
.....
I don' t have a FAP 28C on hand to try this, but most FortiExplorer compatible devices work this way. If you can get into this menu you could reload the firmware via TFTP.
It is pretty unusual to have corrupted firmware problems. I do know there was a bug where under certain circumstances, upgrading image from the GUI (like you were doing) results in that error and I believe it was fixed after the current latest 5.0.7 and before 5.2.0.
If the unit will boot to a prompt, please try loading the firmware via CLI or from the FortiGate " Managed AP' s" section. The latter may be easier.
Let me know if that works. Cheers!
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Sean Toomey, CISSP FCNSP
Consulting Security Engineer (CSE)
FORTINET— High Performance Network Security