Zapier integration error


we are using rocketchat as a group at an University. We want to use Zapier to forward any message written in our private channels to our Slack channel. While the tests in zapier (get messages from RC, send them to Slack) are working, the activation of the zap is not working and gives the following error:
“The app returned “not_authorized”. This usually happens when your Zap is missing a required field or a field value isn’t in a recognized format. We made a request to “our RC url” and received (400) Bad Request.”

I found the following github discussion.

The user is in every channel and according to our support, the oAuth app is also activated.

Do you know any solution for this?