Jenkins pipeline and Gitlab webhooks does not work

[ZeusMinos]

Hi,
Just for reference I am using the latest Jenkins 2.35 with all the latest Gitlab/Pipeline plugins. My Gitlab (Hosted on our own servers) version is 8.13.2.
I have been using webhooks to start other non-pipeline Jenkins jobs and it works ok. I tried to do the same thing using a pipeline Jenkins job and GitLab will return "Hook executed successfully but returned HTTP 500" and nothing happens on the Jenkins side.
My pipeline job just display a message on the console if the job gets triggered.
This is the link used on my Gitlab webhook: http://MyJenkinsServer/gitlab/build_now/MyPipelineJob
Any help is more than welcome
Down below is my pipeline code.

http://paste.ofcode.org/B9VFa5fHxEqbxKqxLEFAki

[amontalban]

Duplicate of #31