A customer wants to use content-type: application/problem+json which is recommend in RFC9457 - Problem Details for HTTP APIs.
The value is invalid in error response settings because + isn't listed in this regex.

  HTTP_HEADER =  /\A[{}\[\]\d,.;@#~%&()?\w_"= \/\\:-]+\Z

Let's take this opportunity to allow the response code field to accept multiline/json text.