When starting the plugin on development, by cloning the repository and doing hpi:run.
When you test the connection at the global configurations or running the slack plugin as a post step. It will fail with various kinds of exception.
There is another kind of exception that I received previously, I'll upload it when I'll see them.

edit: Changing the parent pom to 2.14 solves the issue, but it fails the test because it cant find dependencies.