I have the exact same issue. Attempted update iLO4 from 1.51 to 2.55 but just it just stalls and displays the message:
Checking HP Secure Digital Signature....
If I click on the Group Firmware Option I see the folowing status:
Group Firmware Update
Flashing Firmware Image, please wait ...
Group Firmware Update
1% Complete...
It says like this forever.
I have tried uploaing the iLO.bin file via the webinterface and also tried using the SPP (Service Pack for ProLiant Gen8.1) and boot from a USB device.
Seems like nothin will work.