Casper.local,

I would start with verifying connections, as well as the location the card and cables are installed. After that then clear the hardware log in the iDrac and see if the error persists. Sometimes you can see this when there is just the error logged in the hardware log, that is why I recommended to save and then clear it. This may also be a good time to update the server to current as well. What is the current BIOS installed on the server?

Let me know what you see. 

Thank you for the well crafted and quick response.

I understand that I might have been a bit unclear in explaining what the actual issue is.

The server doesn't boot and I sadly don't have an idrac card.I can only read the error-code from the external display.

Pressing the power-button gives me absolutely no response.

It seems to think there is an invalid riser installation even when there isn't a riser installed. I did go over the cables and tried to reinstall the riser multiple times, even switching out the existing riser with a different one.

I will go over the cabling one more time just to be sure!

Casper

It sounds like it might possibly be a bad riser or motherboard. The Minimum hardware required for the server to post is the following; 

•System board
•Risers
•Processor (CPU1) with Heatsink
•Power Supply
•One stick of memory in dual in-line memory module (DIMM) slot A1
•Control Panel with cable

So if the slot for the riser, or the riser itself, are bad then it wouldn't be able to Post. 

I would recommend if you have another R710, or another known good riser, I would test the slot and the riser separately.

it´s possible to use a generic x16 riser?