How to enable SSL/TLS on the Backup Management Console

Hello,

i'm trying to enable SSL/TLS on my Acronis 12.5 Backup Management Console and i'm failing - it's super frustrating.
I have a self signed certificate and had never any problem with self signed certificates until now.

On the windows machine where the Backup Management Console is installed, i have the certificate and the matching private key stored in:

C:\certificate

I upgraded Acronis Backup & Recovery 12.5 to Version 6 (Build 16428), because after long time of searching on google i founded out, that the SSL connectivity was buggy in Version 4 and got fixed in Version 5. Now i'm up to date.

My api_gateway.json looks like this:

"tls" : {
      "auto_redirect" : false,
      "cert_file" : "C:/certificate/mycert.crt",
      "cipher_suites" : [
         "TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256",
         "TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256",
         "TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384",
         "TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384"
      ],
      "key_file" : "C:/certificate/mycert.key",
      "min_version" : "TLS12",
      "passphrase" : "",
      "subject" : "/C=CH/ST=Schaffhausen/L=Switzerland/O=Acronis/OU=AcronisBackup/CN=AcronisBackup"
   },

What am i doing wrong?
When i connect over HTTPS i get always the message, that it is unsave to connect. The used certificate is not the one i entered in the api_gateway.json, it is like a default one. Why is my own certificate not loading? :-(

Thank you!