Fetch GET 404 npm package throw private Nexus proxy repository

I'm getting a 404 error when I try to "npm install bootstrap" or any other package :
npm http fetch GET 404 https://mynexus/repository/npm-all/bootstrap 167ms

This is my nexus npm-proxy configuration :

I also have configured a nexus npm hosted repository called "npm-all" contaning the npm-proxy.

And this is my .npmrc file :

registry=https://mynexus/repository/npm-all/ 
https-proxy=http://user:password@myproxy:8080 
proxy=http://user:password@myproxy:8080
strict-ssl=false 
prefix=/applis/applis/node 
cache=/applis/applis/node-cache 
email=mymail 
always-auth=false 
_auth="myauth" 

I'm using npm 6.14.7 and node v11.10.0.

Could it be related to a certificate problem ? I checked the logs and I don't see errors talking about certificates.

Anyone has an idea ?

Thanks