Trouble signing outbound emails with DKIM

Hello,

I did follow the different steps on this topic to set the DKIM on my own instance, but at the end of the day (and of the 15 last days indeed), I finish with an empty b field on my email header. I was just wondering if anyone here could help me ?
Regarding the age of this topic, I opened a new one : DKIM : b=signature absent with more details.
I would be really delighted by any help because this one makes me mad.

Thx