Delivery failed for Event Hook

I am currently trying to configure an event hook to synchronize users. I created a servlet in our application which accepts POST and GET where the GET request is the verification.
I created the hook and the verification worked but the delivery of the events always fails.
Connecting with other clients works fine. Are there more steps to debug?

Edit: I already tried what was suggested in Webhook POST fails but endpoint was successfully verified
All certificates are installed in the truststore on the Nginx server

what is the reported error for the delivery attempt?

DisplayMessage: Event Hook Delivery Failed
Reason: Exceeded maximum retry limit of 2

that error points to no response received from your server; is it responding within the 3 seconds timeout period?
you should still receive the event data in this case, but okta would report the delivery as failed because of the lacking response

Your external service’s responses to Okta’s ongoing event delivery POST requests should all be empty, and should have an HTTP status code of 200 (OK) or 204 (No Content).

As a best practice, you should return the HTTP response immediately, rather than waiting for any of your own internal process flows triggered by the event to complete.

Note: If your service does not return the HTTP response within the timeout limit, Okta will log the delivery attempt as a failure.