OK, keeping only the card-mod declaration in the extra_module_url section of frontend seems to fix the problem.
It sounds like I caused this issue because I didn’t fully understood what I was doing. Mea Culpa. 
The first custom add-on I included was the card-mod.
Reading the readme, I understood that I had to add the declaration in frontend/etra_module_url.
I simply assumed that I had to do the same modification for all the other custom add-on.
I’ll monitor the behavior, but so far so good.