Place an external break point in method IF_HTTP_EXTENSION~HANDLE_REQUEST of class CL_HTTP_EXT_ITS:
callmethod server->set_session_stateful_via_url
exporting
stateful = if_http_server=>co_enabled
changing
rewrite_url = path. " <-- external break point
What is the value of path? The method gets called twice, first for the GET and the second time for the POST. When called the first time the method builds an HTML response page containing a form that will be automatically posted which results into the POST.