Thanks for the update, I know this is totally my fault. So thanks for taking the time for that.
After some further digging, it now does appear like Playtomic relies upon POST to send data (eg leaderboards). The funny thing is I'm still not getting the error. When I run the project it accepts POST and OPTIONS just fine which is how it slipped by me unnoticed.
So unfortunately while it looks like the update will make the error message less jarring, as long as the request is rejected the plugin remains broken.
Correct me if I'm wrong, it should work if they export & upload their projects to an external server that supports POST etc? Guess I'll just have to update the tutorial to with another step to 'not-so-easily add leaderboards'