I finally figured out, how it works. If anyone should come across the same problem, here it is:
When I add a Pipedrive trigger node to my workflow, n8n uses the Pipedrive API to add a new webhook. This API is only accessible if the OAuth2 settings in Pipedrive include the access scope “Administer account”.
If I don’t want to give such a powerful access scope to the app, I have to create the webhook in Pipedrive manually (via Settings > Tools and apps > (Tools) > Webhooks) and then create a generic webhook node in n8n instead of using the Pipedrive trigger node.