Ah, I'm seeing the issues here.
So, 308986 is not the id of the custom field. 308986 is the id of the value that is checked within the custom field. The id of the custom field you are looking at is 154075. That final screenshot should be returning a 404 not a 500 (that's a bug on our end that will be fixed later today) since that is not a custom field id in the url.
Secondly, you're not actually doing the filters correctly. They do not go in url. Whenever you see json in the API v3, that goes in the body of the call, not the url.
Here is a screenshot of what you'd want to do in postman to get the results you want:
