Mattermost integration: Requests to localhost are not allowed

using gitlab EE I run mattermost & gitlab on the same server.

so far I have:

Mattermost & gitlab each run fine on it’s own, but if I add a Mattermost Integration in a gitlab project, I’ get the following error message on “Test/Save”:

Validations failed. Webhook is blocked: Requests to localhost are not allowed 

Could anyone explain what I’m missing?
thanks in advance