Adding SSH-key through curl: Fingerprint cannot be generated

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I’m writing an bash script that will add an SSH-key to an in-house Gitlab server. Below is the code:

userToken=$(curl http://gitserver/api/v3/session --data 'login=*****&password=****' | jq '.private_token')
userToken=$(echo "$userToken" | tr -d '"')

# Below key is for testing, will use output of cat ~/.ssh/id_rsa.pub later on
sshKey="ssh-rsa AA*********L ****@***com"

curl --data "private_token=$userToken&title=keyName&key=$sshKey" "http://gitserver/api/v3/user/keys"

But I get this response:

{
  "message": {
    "fingerprint": [
      "cannot be generated"
    ]
  }
}

But when I mannualy insert the same key through the webbrowser, everything is just working fine…

So what am I doing wrong? Do I miss an option or is there somewhere an setting I’m unaware of?