Hi @ukumito,
I’m sorry you have experienced difficulties.
Is /agenda/ the slug for your main events view, or have you tried to add a custom view?
Can you try visiting the Settings → Permalinks admin screen – and then try accessing your events view? This can sometimes clear out and refresh the rules WordPress uses to route requests.
Last but not least, you indicated having completed our standard troubleshooting steps. What was the outcome of this? Did the same problem still occur even with no other plugins and only a default theme in place?
Let me know how you get on 🙂