Ok, i've setup a fresh scm-manager installation with a fresh jenkins installation. I've installed both services on the same machine with different ports. SCM-Manager uses port 8081 and Jenkins uses port 8080. Here are my steps:
- download scm-manager 1.40
- configure port to 8081
- create git repository named test
- download jenkins
- start jenkins
- install git-plugin plugin
- restart jenkins
- create freestyle jenkins job named test
- choose Git as "Source Code Management"
- press "Add" button below the credentials field
- choose Kind: "Username with password", Scope: Global, Username: scmadmin, Password: scmadmin
- select scmadmin/****** from the credentials checkbox
- click save
The above setup worked without any problems. The interesting is, i've tried to reproduce the error message you get. But i've never seen error 403. It sounds like your jenkins does not talk to the scm-server, it is possible that you jenkins installation talks to a different server? Do you use a reverse proxy? Could you please describe your whole setup (machines, plugins, users, etc.)?
Sebastian