Hi @Florent,
I think I guess why you are facing this issue.
When creating a development environment, host have to be explicitly localhost and be accessible via http. This is the only host Forest Admin allows without having to set 
https , for security reasons; in a regular process, your development environment have to be local and accessible through localhost. Then, after this local environment works, you would be able to create other environments (prodution, staging, etc.). But still they will require to be accessible through https only.
Could you consider trying to install your backend on localhost first? 