Server error on /oauth/userinfo

Hi!
I’m trying to make API calls according to this how-to article
https://developers.docusign.com/platform/auth/authcode/confidential-authcode-get-token/

But I’m stuck on “Step 3. Get your user's base URI” - the https://account-d.docusign.com/oauth/userinfo url always response with:

{
	"error": "internal_server_error",
	"reference_id": "d63dceae-72c6-4d27-9218-3b0fd58b2c30"
}

Authorization header is added as described in the docs -

Bearer somelongCode......Provided

IDK why it’s happening because I’m able to get Docusign access token from step 2 - that’s means that credentials set properly. 
So what I’m doing wrong?
Thanks!