webhook for slack returns error 400 no_text

Summary Slack-Gitlab integration by webhook gives error 400 'no_text' upon testing

I know the webhook is okey, cause a simple curl post works fine. It's only from the integration with gitlab that messages are not arriving. Also slack integrations is turned on and works fine.

Steps to reproduce Just add a custom webhook via "Add webhook" and try to test it. https://docs.gitlab.com/ee/user/project/integrations/webhooks.html

THANKS!