Mattermost OAuth - InternalOAuthError: Failed to obtain access token

Hello,
I'm trying to configure Mattermost- OAuth configuration with docker deployment. I get error message after the callback :
InternalOAuthError: Failed to obtain access token.

It seem like codimd cannot manage the access_token. But I don't know how to find what the problem is.

I use " ... /auth/mattermost/callback" as callback url in Mattermost configuration (found it in the code).
The access-token url can be accessed with wget withing the codimd container.
Mattermost is a local instance, with a valid SSL certificate. This is a "joker" certificate.

I don't know if it is a Mattermost configuration problem, or a connection problem between codimd and mattermot, or a format probleme
I haven't succeded to enable more precise debug informations.