Hi Sébastien!
Assuming the slug is correct, there may be another plugin or your theme changing what events are returned.
You can determine which plugin or theme is causing the issue with the following steps:
1. Ensure you have a backup of your site in case there are any issues when disabling/enabling plugins or your theme
2. Disable all plugins (except the calendar plugin and this plugin), and switch to a default theme like Twenty Sixteen then verify the issue is resolved
3. Re-enable your plugins and switch back to your previous theme one by one verifying if the issue is still occurring, and note which one causes it to break
Let me know which plugin/theme causes the issue and I may be able to come up with a fix, but you’ll also need to contact the theme or plugin author and have them fix the issue.
Cheers!