spi-event-listener returns a white page in case of POST error

I'm using Keycloak 17.0.1 and I successfully deployed the spi-event-listener on my testing server. Unfortunately, if I artificially put an error in the URI (so response return 404), The user is getting a white page instead having a proper error.

Sorry, I'm newbie in Java, I'm spend few hours already but don't understand how I can fix this issue.

Thanks