Thank you all for your replies ! Really appreciated. The good news is that the issue seems to be fixed now (see below)
To clear up confusion about the Midi device, the Iconnectivity MIO is just a 30€ USB Midi to Din converter and doesn’t have any audio pass-thru feature unlike the IconnectMIDI2+ or 4+ made by the same manufacturer. It’s not a Metric Halo either, only a simple converter 
These are the result of cat /proc/asound/modules in two different scenarios:
A . Norns booted without the MIO already connected and connected after
0 snd_soc_rpi_proto
1 snd_usb_audio
B. Norns booted with the MIO already connected. (causing the error)
0 snd_usb_audio
1 snd_soc_rpi_proto
I’m running this on a RPI3B+, Raspbian Stretch and the soundcard is not an external USB card but a WM8731 like Fates. The onboard RPI soundcard is blacklisted. The system configuration and install process is almost identical as Fates. (It’s mostly the config and install process based on the work of @okyeron @TheTechnobear and others )
Yes, the MIO enumerates as a USB audio interface, therefore causing a problem with the main soundcard (not the onboard RPI audio but a WM8731 like Fates) It seems to be the case with other Midi devices as @okyeron mentioned. Yes, we share the same configuration.
Modifying alsa-base.conf as you’ve suggested above has fixed the issue, thanks a lot. I think I remember we used to do something similar from the very first « Norns on Raspi install instructions », then this step became optional. I’ll update the install scripts.
And today I will test the new multiport devices functionality with an IconnectMIDI2 this time, it’s great to have this feature ! 
Thanks again
