Hi @Flowfinder, I am not using SAP AppGyver but I hope you don’t mind me wildly guessing here 
Is there a chance this service uses the browser for it’s test requests rather than talking to n8n from its server? If so, this request might be subject to CORS: Cross-Origin Resource Sharing (CORS) - HTTP | MDN. You should see a more descriptive error message in your browser console if that’s the case.
n8n doesn’t handle the OPTIONS requests required for CORS unfortunately, you might want to leave a vote on the respective feature request though: Add ability to set CORS allow-list in n8n webhooks