With Spring Boot Webflux reactive application, to debug and identify the unexpected exception thrown from a handler while handling http request, put a debug / break point in the following method of the class. This will quickly reveal the exception along with stacktrace with your favourite IDE Class: org.springframework.web.reactive.DispatcherHandler Method: handleRequestWith Have a breakpoint in the line after Mono<HandlerResult> resultMono = adapter.handle(exchange, handler) and evaluate as resultMono.block() Happy Debugging !!!