Hi, i'm not an expert on this but when I used similar codes to retrieve tokens I have to pass the client_id, client_secret and the refresh token.
Every API has it's own way to get access but I think you have to pass some kind of credentials, ie:
FROM JSON (wrap on) "root"
WITH CONNECTION (
URL "$(vRequestURL)"
This "vRequestURL" is a variable than contains the url with all the needeed parameters the API requires to return the access_token.
Maybe something has expired and it retuns a login or error page, and this page is what the rest connector can't parse.
I'm just guessing, you have to look in the documentation of that API how to get the token... or wait to another community user with more knowledge of this than me.