Room suddenly gets unusable

[pdelacroix]

Description

A room suddenly stops accepting messages from federation or from client API (C2S). Happened on 1.153.0, I updated to 1.156.0 in case it fixed it, it didn't.

Steps to reproduce

• very normal conversation

Homeserver

another homeserver

Synapse Version

1.156.0

Installation Method

Docker (matrixdotorg/synapse)

Database

Single Postgres 17 server, previously restored from a backup but a long time ago, probably before this room existed

Workers

Multiple workers

Platform

Docker on Debian on bare metal

Configuration

No response

Relevant log output

2026-07-15 13:59:47,521 - synapse.http.server - 147 - ERROR - PUT-328- Failed handle request via 'RoomSendEventRestServlet': <XForwardedForRequest at 0x7531f05120d0 method='PUT' uri='/_matrix/client/v3/rooms/[room_id]/send/m.room.encrypted/[transaction_id]' clientproto='HTTP/1.1' site='8008'>
Traceback (most recent call last):
  File "/usr/local/lib/python3.13/site-packages/synapse/http/server.py", line 335, in _async_render_wrapper
    callback_return = await self._async_render(request)
                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.13/site-packages/synapse/http/server.py", line 576, in _async_render
    callback_return = await raw_callback_return
                      ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.13/site-packages/synapse/rest/client/room.py", line 506, in on_PUT
    return await self.txns.fetch_or_execute_request(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    ...<8 lines>...
    )
    ^
  File "/usr/local/lib/python3.13/site-packages/twisted/internet/defer.py", line 1187, in __iter__
    yield self
  File "/usr/local/lib/python3.13/site-packages/twisted/internet/defer.py", line 1857, in _inlineCallbacks
    result = context.run(gen.send, result)
  File "/usr/local/lib/python3.13/site-packages/synapse/rest/client/room.py", line 481, in _do
    ) = await self.event_creation_handler.create_and_send_nonmember_event(
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        requester, event_dict, txn_id=txn_id
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File "/usr/local/lib/python3.13/site-packages/synapse/handlers/message.py", line 1109, in create_and_send_nonmember_event
    return await self._create_and_send_nonmember_event_locked(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    ...<11 lines>...
    )
    ^
  File "/usr/local/lib/python3.13/site-packages/synapse/handlers/message.py", line 1156, in _create_and_send_nonmember_event_locked
    event, unpersisted_context = await self.create_event(
                                 ^^^^^^^^^^^^^^^^^^^^^^^^
    ...<9 lines>...
    )
    ^
  File "/usr/local/lib/python3.13/site-packages/synapse/handlers/message.py", line 750, in create_event
    event, unpersisted_context = await self.create_new_client_event(
                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    ...<10 lines>...
    )
    ^
  File "/usr/local/lib/python3.13/site-packages/synapse/util/metrics.py", line 180, in measured_func
    r = await func(self, *args, **kwargs)
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.13/site-packages/synapse/handlers/message.py", line 1415, in create_new_client_event
    event = await builder.build(
            ^^^^^^^^^^^^^^^^^^^^
    ...<4 lines>...
    )
    ^
  File "/usr/local/lib/python3.13/site-packages/synapse/events/builder.py", line 303, in build
    auth_event_ids.remove(create_event_id)
    ~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
ValueError: list.remove(x): x not in list

Anything else that would be useful to know?

No response