Hey, I saw lots of blog post regarding Bubbles API & OAuth2 Custom Token, but after several attempts and reading lots of blogs, I still do not find the issue.
As you can see the request works with Curl and with Postman:
I suggest to send the call to service like requestbin from both api connector and postman and compare each request. I think that your issue could be related to username and password url encoding.
Thx for the hint using requestbin, I was also thinking about that approach, but not yet found an easy to use tool. Actually, you are right, it is an encoding issue, in this case of special characters. Here the part of the payload shown by requestbin:
So, using the already encoded payload in Bubble its seems working. Question remains, is there a way to convince Bubble.io to perform the encoding itself?