Note that I use dev-master version.
I went in debug mode, this is the full debug log :
POST https://www.xxxxxxxxxxxxxx.fr/api/chatmessages/0
Flarum\Http\Exception\MethodNotAllowedException: POST in /vendor/flarum/core/src/Http/Middleware/ResolveRoute.php:60
Stack trace:
#0 /vendor/laminas/laminas-stratigility/src/Next.php(51): Flarum\Http\Middleware\ResolveRoute->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#1 /vendor/flarum/core/src/Http/Middleware/SetLocale.php(51): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#2 /vendor/laminas/laminas-stratigility/src/Next.php(51): Flarum\Http\Middleware\SetLocale->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#3 /vendor/flarum/core/src/Http/Middleware/AuthenticateWithHeader.php(58): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#4 /vendor/laminas/laminas-stratigility/src/Next.php(51): Flarum\Http\Middleware\AuthenticateWithHeader->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#5 /vendor/flarum/core/src/Http/Middleware/AuthenticateWithSession.php(31): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#6 /vendor/laminas/laminas-stratigility/src/Next.php(51): Flarum\Http\Middleware\AuthenticateWithSession->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#7 /vendor/flarum/core/src/Http/Middleware/RememberFromCookie.php(52): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#8 /vendor/laminas/laminas-stratigility/src/Next.php(51): Flarum\Http\Middleware\RememberFromCookie->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#9 /vendor/flarum/core/src/Http/Middleware/StartSession.php(61): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#10 /vendor/laminas/laminas-stratigility/src/Next.php(51): Flarum\Http\Middleware\StartSession->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#11 /vendor/flarum/core/src/Api/Middleware/FakeHttpMethods.php(29): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#12 /vendor/laminas/laminas-stratigility/src/Next.php(51): Flarum\Api\Middleware\FakeHttpMethods->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#13 /vendor/flarum/core/src/Http/Middleware/ParseJsonBody.php(28): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#14 /vendor/laminas/laminas-stratigility/src/Next.php(51): Flarum\Http\Middleware\ParseJsonBody->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#15 /vendor/flarum/core/src/Http/Middleware/HandleErrors.php(57): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#16 /vendor/laminas/laminas-stratigility/src/Next.php(51): Flarum\Http\Middleware\HandleErrors->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#17 /vendor/flarum/core/src/Http/Middleware/InjectActorReference.php(25): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#18 /vendor/laminas/laminas-stratigility/src/Next.php(51): Flarum\Http\Middleware\InjectActorReference->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#19 /vendor/laminas/laminas-stratigility/src/MiddlewarePipe.php(76): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#20 /vendor/middlewares/request-handler/src/RequestHandler.php(84): Laminas\Stratigility\MiddlewarePipe->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#21 /vendor/laminas/laminas-stratigility/src/Next.php(51): Middlewares\RequestHandler->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#22 /vendor/middlewares/base-path-router/src/BasePathRouter.php(101): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#23 /vendor/laminas/laminas-stratigility/src/Next.php(51): Middlewares\BasePathRouter->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#24 /vendor/laminas/laminas-stratigility/src/Middleware/OriginalMessages.php(36): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#25 /vendor/laminas/laminas-stratigility/src/Next.php(51): Laminas\Stratigility\Middleware\OriginalMessages->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#26 /vendor/middlewares/base-path/src/BasePath.php(73): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#27 /vendor/laminas/laminas-stratigility/src/Next.php(51): Middlewares\BasePath->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#28 /vendor/flarum/core/src/Http/Middleware/ProcessIp.php(24): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#29 /vendor/laminas/laminas-stratigility/src/Next.php(51): Flarum\Http\Middleware\ProcessIp->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#30 /vendor/laminas/laminas-stratigility/src/MiddlewarePipe.php(76): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#31 /vendor/laminas/laminas-stratigility/src/MiddlewarePipe.php(65): Laminas\Stratigility\MiddlewarePipe->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\EmptyPipelineHandler))
#32 /vendor/laminas/laminas-httphandlerrunner/src/RequestHandlerRunner.php(96): Laminas\Stratigility\MiddlewarePipe->handle(Object(Laminas\Diactoros\ServerRequest))
#33 /vendor/flarum/core/src/Http/Server.php(44): Laminas\HttpHandlerRunner\RequestHandlerRunner->run()
#34 /public/index.php(26): Flarum\Http\Server->listen()
#35 {main}