These systems are very picky when it comes to aftermarket fan support.

There's only a few select brands and models that they accept without throwing boot error codes.

I use Noctua's NF-A12X25 PWM fans as replacements on my R10, with no issues at all.

The only drawback when using aftermarket fans is the fan speed readout under AWCC in %. It will only go up to about 50%, since the original fans are around 4,000 RPM and the Noctua's are 1,800 RPM. For some reason AWCC has the % hard coded...

Besides that they work without an issue.

You can't use any RGB fans and the R12 only boots without error with non rgb ML120 Pros 

Do you know if Noctua's also work in the R12? I know they work in the R10, not sure about the R12?

The MF120s 3-pack was a gift.
I didn't know they wouldn't work with the R12.

Last evening, when I got the problem, I thought about there might be some kind of compatibility issue and I tried to search on the net, but nothing popped out.
Perhaps I missed them... (I was tired and frustrated -_- )
Anyway, good to know that.
I'll figure out how to arrange the stock fans to fit them inside the case and not hanging out of it.

Thanks for your answers

Noctuas work fine in the R12, however you get the bios fan check screen just skip it. RGB fans throw the boot loop issue

I'm using the Arctic P12 PWM fan without any boot issue as well.