I tried getting the Create Draft Post API to work as well, but it failed when run from my site. I did get the “API tester” to work though. The only difference was the addition of the fieldsets parameter, but when I tried adding that to my site code, it still failed. Here is the POST body that I used when it worked in the API Tester.
{
"draftPost": {
"title": "A Blog Test"
},
"publish": false,
"fieldsets": []
}
I’m not sure why the API tester didn’t work for you, but I didn’t get the same 500 error that you reported…