Staging system - Create contact - HTTP response code: 404

I'm trying to create a new contact in the staging system (via Java post functionality), but I get back
HTTP response code: 404
HTTP response body:
HTTP response headers: {content-length=[0], content-type=[application/json], date=[Tue, 28 Oct 2025 09:56:35 GMT], referrer-policy=[strict-origin-when-cross-origin], server=[nginx], strict-transport-security=[max-age=63072000; includeSubDomains], vary=[Origin], x-frame-options=[SAMEORIGIN], x-permitted-cross-domain-policies=[none], x-request-id=[f22ed584-a586-4313-bbd6-eb95277563f5], x-runtime=[0.060462]}
The response was logged back to me at 28.10.2025 10:56:42.365 CET.

Could you please indicate what are we missing (what is "Not Found" on the server side)?
Below is the request for that call (URL, headers, json data for the contact). Could it be because of the special chars in the address/city "Schloßschmidstrasse5" /"München"?

Request{method=POST, url=https://app-staging.b2brouter.net/accounts/77640/contacts, headers=[X-B2B-API-Key:b4a5....bfe8, Accept:application/json, Content-Type:application/json, User-Agent:OpenAPI-Generator/2025-10-13/java]}

{"contact":{"tin_value":"DE304950997","tin_scheme":"9958","name":"Axxxx GmbH Xxxxxx","email":"[email protected]","address":"Schloßschmidstrasse5","postalcode":"80639","city":"München","country":"DE","language":"de","currency":"EUR","is_client":true,"is_provider":false,"transport_type_code":"peppol","document_type_code":"xml.ubl.invoice.bis3","pin_value":"16701122-0005-91","pin_scheme":"9958","payment_method":0,"terms":"custom","public_sector":false}}

Thanks and kind regards