Legal characters for usernames

Currently, it's required to be alphanumeric (A to Z - upper or lower - and 0-9). The issue with it returning InvalidParams instead of InvalidUsername is because our system does early-out checks on the input parameters to all calls, with the alphanumeric check for usernames being at that stage. Since changing that now would be a breaking change for anyone using that error, we can't really change it for the current API calls, but we can consider this as a change for the future. However, you do get a consistently reliable errorDetails that tells you that the username has invalid characters:

{
    "code": 400,
    "status": "BadRequest",
    "error": "InvalidParams",
    "errorCode": 1000,
    "errorMessage": "Invalid input parameters",
    "errorDetails": {
        "Username": [
            "Username contains invalid characters."
        ]
    }
}