[Arhowk]

Are you using the command line om51_flash.exe tool? If not, please elaborate on what you're doing, If so...

Firstly, make sure your ethernet is connected to the one farthest on the ethernet and the robot is turned off.
Second, make sure all of your network interfaces are disabled through the network manager. Also make sure that this ethernet port has a dynamic IP and that you have no firewalls on.
Third, when you run the flash tool (in Administrator mode) there should be only one option to flash. Thicis should be 'Ethernet', 'Local Area Connection', or something

When you get to the step where it says "Listening for radio to connect", enable the robot. It should take about 10seconds before the command prompt starts printing text again.

If you're still encountering errors, than this means that the flasher tool is not able to communicate in the bootup period to flash software. This could be due to a wide variety of things, most of which I instruct you to avoid in the above steps. In the period where the router is booting up, I'd try to ping the router if it is returning handshakes before it is totally booted up. If so, that would indicate an issue in the program's ability to talk to the router, in which case its probably a silly windows thing like its not in administrator mode, firewall is blocking its attempted network calls, etc.. If you can't ping to it before the router is totally booted up, than its more likely a PC-side issue than a router-side issue. Is your LAN port constantly scanning itself for ethernet? Do you get a connected network within a few seconds of turning on the router?