Within a rest request I send another request to a webservice using Spring WebClient and want to return the result to the caller:

return webClient.post()
  .uri(url)
  .body(...)
  .retrieve()
  .bodyToMono(String::class.java)
  .map { ResponseEntity.ok(it) }

Now it that webservice returns a HTTP error status code I receive the following error:

java.lang.IllegalStateException: Only one connection receive subscriber allowed.

When a status 200 code is returned, no error is thrown.

I also tried checking the status and throwing an exception myself, but the behavior is still the same:

return webClient.post()
  .uri(url)
  .body(...)
  .retrieve()
  .onStatus(HttpStatus::isError) { clientResonse ->
     clientResonse.bodyToMono(String::class.java)
        .map { RuntimeException(it) }
  }
  .bodyToMono(String::class.java)
  .map { ResponseEntity.ok(it) }

What am I doing wrong?

The code samples are in Kotlin but I assume every Java dev can read it :)