Anders Aagaard opened SPR-16073 and commented

See https://gist.github.com/andaag/f30aeff3bda499bb159f50649a0eb353

When returning a Mono<ResponseEntity>, and the returned ResponseEntity has a custom http code, spring throws an java.lang.IllegalArgumentException: No matching constant for [555]

Affects: 5.0 GA

Issue Links:

• RestTemplate doesn't consistently tolerate unknown HTTP status codes [SPR-15978] #20529 RestTemplate doesn't consistently tolerate unknown HTTP status codes
• ResponseEntity should allow the usage of unassigned HTTP status codes [SPR-14205] #18779 ResponseEntity should allow the usage of unassigned HTTP status codes
• ServerResponse should allow the usage of unassigned HTTP status codes [SPR-16366] #20913 ServerResponse should allow the usage of unassigned HTTP status codes
• No support for non-standard HTTP status codes in reactive ClientHttpResponse [SPR-16748] #21289 No support for non-standard HTTP status codes in reactive ClientHttpResponse

Referenced from: commits 3890d4c