Hi, Kévin.
>I've followed the documentation, and registered an OpenId Connect Client in the UI interface
Could you provide a link to the doc you followed?
>I've then created a simple client to test this:
Please note that community (free) users support doesn't cover custom client development. Consider using [one of the officially supported clients](https://www.gluu.org/blog/limiting-openid-connect-community-client-support/) for your first time with Gluu, and trying to experiment with any non-standard setups only after you've read [the spec](http://openid.net/specs/openid-connect-core-1_0.html) and studied how it works in practice. You could follow our [mod_auth_openidc doc](https://gluu.org/docs/ce/3.1.2/integration/sswebapps/openidc-rp/), for example.