hm, I have a problem with the actual backports synapse. I installed it this noon, but it throws:

synapse.metrics.background_process_metrics: [user_directory.notify_new_event-1981] Background process 'user_directory.notify_new_event' threw an exception
                                            Traceback (most recent call last):
                                              File "/usr/lib/python3/dist-packages/synapse/metrics/background_process_metrics.py", line 208, in run
                                                return await maybe_awaitable(func(*args, **kwargs))
                                              File "/usr/lib/python3/dist-packages/synapse/handlers/user_directory.py", line 110, in process
                                                await self._unsafe_process()
                                              File "/usr/lib/python3/dist-packages/synapse/handlers/user_directory.py", line 159, in _unsafe_process
                                                self.pos, room_max_stream_ordering
                                              File "/usr/lib/python3/dist-packages/synapse/storage/databases/main/state_deltas.py", line 51, in get_current_state_deltas
                                                prev_stream_id = int(prev_stream_id)
                                            TypeError: int() argument must be a string, a bytes-like object or a number, not 'NoneType'

now. reportbug offers me testing and unstable for reporting. So how do I report a backports bug?