Hi @Martijn3293, welcome to the community!
I am sorry to hear you’re having trouble here. Can you confirm on which domain you are seeing the 404 status? Would that be an n8n tunnel URL (ending with …n8n.cloud)?
If so, I wonder if the tunnel URL might have changed since you added them to Zoho (these URLs aren’t guaranteed to be permanent or have a 100% availability). Could you double-check that?
Your OAuth2 settings look fine to me, I was using very similar settings for another Zoho service recently (these ones).