iDRAC6 firmware update failed

Hi,

I just got an old PowerEdge R710 server which I wanted to update the iDRAC6 firmware but no matter what and how I try it doesn't work.
This is what I have:
PowerEdge R710
BIOS Version 6.4.0
iDRAC6 - Enterprise
Firmware Version 1.10.13
Hardware Version 0.01

I tried to update using the WebGUI trying all different firming.d6 versions (from eg. http://ftp.us.dell.com/FOLDER00681749M/1/Looking%20for%2011G%20iDRAC%20MONOLITHIC%20Releases.htm)

I also tried using TFTP server and even installed Windows Server 2008R2 and 2016 TRAIL versions and ran the .exe files to upgrade iDRAC6.

But no luck. It always stopps with an error like 'failed to update' or similar without any hint what actually the reason might be.

Does anyone have a clue what I could try?

Thanks in advance,
Matt