Custom Button in Google Sheet doesn't run: error code 400

Hi there,

I put up a scenario, that sends e-mails depending on changes made in Google Sheet. The scenario runs if I schedule it at regular intervals or I if I click the Run once button.

But here is a thing: I want to run this scenario on-demand by a client who does not use Make, just have access to the Google Sheet. So I followed this tutorial, and set up a custom button using “Make for Google Sheets” extension and Apps Script.
https://www.make.com/en/help/app/google-sheets#add-a-custom-button-in-a-sheet-to-trigger-a-scenario

The problem that confuses me is this: even if I insert a trigger module at the beginning of the scenario (Google Sheet Watch Changes or Custom Webhook), it only works, when I test it. For example: if I use the Custom Webhook, and click on “Redetermine data structure”, then it begins to “listen” to incoming data. If I click on my custom button in Google Sheet after that, it runs perfectly: all the scenario is completed. But after this test mode, if I schedule the scenario “immediately” or “on demand”, the same process stops.

If I click on the same custom button in Google Sheet, instead of 200 code, I receive a 400 error code with the following message: “There is no scenario listening for this webhook”. I checked the logs and details: my failed requests are in a queu, but I can’t find out what’s the problem.

How can I make sure that my scenario runs immediately after someone hits the custom button in my Google Sheet? I really need a helping hand in this so thanks for any kind of tips&hints!