Thanks for the very quick answer, MC!
And sorry for the not so quick reply.
I was thinking on how to explain this, so I realized a video speaks better than I can.
https://youtu.be/4rtKegyubz8

It seems like an erratic signal.
I am only playing one audio file constantly, but because the interface is only erroneously sending data via the AES out, and not real audio, it is an erratic signal that seems to start and end abruptly every few seconds.
This causes the problem that I cannot operate my ADI because the messages don't stop popping up.

This is not RME's fault at all, but I contacted the vendor of the other interface, and they say they no longer support it (lesson learned there).

If anything can be done, that would be more than amazing. Thanks!

Hi, KaiS.
Thanks for the possible solution.

That goes against my setup, which I will explain in a sec, but I did test it.
With that setup, I no longer get the message, but it did create another problem not worth discussing here.

Now, the real reason I cannot go for that setup is:

My other interface, Antelope Discrete SC 4, has 4 inputs. It has no AES outputs, and I am using a Lynx cable that is made for S/PDIF to AES.
It does have ADAT.
Everything can be routed.

The reason I bought it was merely to have 4 preamps that I could connect at once to my ADI.
So now they are connected as follows, and I use my ADI in Multichannel mode:

ADSC4 input 1 and 2 go out to ADAT out, this is being used in my ADI Optical input as ADAT.
ADSC4 input 3 and 4 go via S/PDIF out to AES in.
The analogue inputs of my ADI are tied to another stereo device, and the analogue outputs of the ADSC4 are exclusively for monitoring, connected directly to my monitors.

I have made sure to route the unit so the S/PDIF and ADAT outputs, all digital outputs, are muted when testing this issue. It will still send some type of signal, which is not audio, and is noted as Emphasis by the ADI.

So the setup is basically a 4x4 connection, and it is so just so I do not have to switch cables here and there and I can assign specific things to each input/output, since I work alone and I am not recording other people, at least not simultaneously, but I do use multiple devices.

Now, because of that I can only think of 3 solutions:
1) Having Antelope fix the firmware: They said already they won't fix a discontinued product (even if discontinued very recently, I bought it maybe a month before official discontinuation).
2) The ADI to be able to input ADAT (and or AES) via the Optical port and allow for the S/PDIF ports in the breaker to be used for S/PDIF separately and at the same time as the optical. I doubt is possible, due to the design of the unit (no expert here, but I figure it would have been done if possible).
3) Add an option to disable the message, as it is harmless.

Of course, if there is any other real option anyone can think of, I will love you forever .
I am not the most proficient guy when it comes to digital audio, so I apologize for my lack of knowledge.

Now, if none of these are an option, I will have to sell the ADSC4 and purchase another secondary interface, but I really hope that will not be necessary as the preamps are quite good and other than the message I have no issues with it.
(Even if it only has analog outputs, I actually wanted to get the RME Quadmic but I realized it has no Hi-Z. A Hi-Z enabled version would be killer, and more so if it has some type of digital output).

Thanks for all the help.