Error "Deserialization problem: Content is not allowed in prolog.; " When Trying To Sign-in To Tableau Server By REST API With Personal Access Token

Issue

When trying to sign in to Tableau Server by REST API with personal access token which is valid, Got an error below and can not sign in to Tableau Server.

{
    "error": {
        "summary": "Bad Request",
        "code": "400000",
        "detail": "Deserialization problem: Content is not allowed in prolog.; "
    }
}

There are error messages in the log of vizportal like below.

2021-07-06 16:37:39.709 +0900 (-,-,-,YOQIQ55foCKlIK9GeduMiwAAAh4,0:cdc771f:17a73dc564a:20f9) catalina-exec-132 vizportal: INFO com.tableausoftware.app.vizportal.LoggingInterceptor - Request received: /3.7/auth/signin
2021-07-06 16:37:39.803 +0900 (-,-,-,YOQIQ55foCKlIK9GeduMiwAAAh4,0:cdc771f:17a73dc564a:20f9) catalina-exec-132 vizportal: INFO com.tableausoftware.domain.user.service.RefreshTokenService - Unable to redeem refresh token
com.tableausoftware.domain.exceptions.InvalidRefreshTokenException: java.lang.IllegalArgumentException: Illegal base64 character 20 (errorCode=56)
at com.tableausoftware.domain.user.service.RefreshTokenService.getAndVerifyRefreshToken(RefreshTokenService.java:646)

Environment

Tableau Server 2020.1.2

Resolution

Check the setting value of HTTP Request Header - ContentType.
If the value for HTTP Request Header - ContentType is not application/json or application/xml,
Should use application/json or application/xml for Content-Type of REST API Request.

Cause

Setting the Content-Type heading to an incorrect type, such as application/x-www-form-urlencoded, can result in unexpected corruption of your posted data.