Token validation failed. Unable to validate issuer

Question

I'm using CustomAuthorizationAttribute: AuthorizeAttribute to validate an access token. Below is the code snippet. Same code works fine in local. But when it is deployed to IIS, it throws an exception:

Unable to validate token. validationParameters.Issuer is null or whitespace and validationParameters.Issuers is null or empty. (using log for exception)

Token validation parameters:

return new TokenValidationParameters()
           {
               ValidateLifetime = true,
               ValidateAudience = true,
               ValidAudience = "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxx",
               ValidateIssuer = true,
               ValidIssuer = "https://login.microsoftonline.com/xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxx/v2.0",
                        
               ValidateIssuerSigningKey = true,
               IssuerSigningKeys = openidconfig.SigningKeys,
           };

private bool ValidateToken(string token)
{
    var validationParameters = GetValidationParameters();
    var tokenHandler = new JwtSecurityTokenHandler();

    try
    {
        SecurityToken validateToken;
        tokenHandler.ValidateToken(token, validationParameters, out validateToken);
        return true;
    }
    catch (SecurityTokenValidationException ex)
    {
        WriteLog(ex.Message);                
        return false;
    }
}

protected override bool AuthorizeCore(HttpContextBase httpContext)
{
    var authtokenHeader = httpContext.Request.Headers["Authorization"];
    WriteLog(authtokenHeader);

    if (!string.IsNullOrEmpty(authtokenHeader) && 
        authtokenHeader.StartsWith("Bearer", StringComparison.OrdinalIgnoreCase))
    {
        string accessToken = authtokenHeader.Substring("Bearer".Length).Trim();

        if (ValidateToken(accessToken))
        {
            return true;
        }

        return false;
    }
    else
    {
        WriteLog("User not authorized - Auth token is empty or null");
        return false;
    }
}

web.config authentication mode is none. Is there anything I need to add to the web.config?

<system.web>
    <compilation debug="true" targetFramework="4.8" />
    <authentication mode="None" />
</system.web>

Any help would be appreciated. Thanks in advance.