"Trusted server" refuse to federate (icon stays yellow all the time)

[e-raser]

I recommend to take a look at this topic:

Federation: cannot add trusted server

Hi everybody, I never got to get federation running. All indipendent NC-11 servers, partly upgraded, partly newly installed. Federation works on none of them. The trusted server icon stays forever yellow. The log says: GuzzleHttp\Exception\ClientException: Client error response [url] https://servername/ocs/v2.php/apps/federation/api/v1/request-shared-secret?format=json [status code] 400 [reason phrase] Bad request Any ideas?