I was doing some testing in my local environment and I think I found a bug in your PHP plugin. If you change the slug driver of Users to 'id' instead of 'default' the code will make an incorrect request to the forum API.
However, if you restore the 'default' value to the database again, it works without problems.
The error is
404 Not Found with a URL like:
Let me know if you need more details to reproduce it.