I wanted to solve this puzzle. 😂
All I can assume that caused the problem is once you updated Flarum to v1.7.1, and you got the error. You went through troubleshooting common fixes. Something that you tried in that article must have worked. But you left the debug mode on by accident.
This brings be back to my question, what actually solved this error?:
Flarum\Http\Exception\MethodNotAllowedException POST?
😂
But I am glad it's solved for you. 🙂