diff --git a/changelog.d/15323.bugfix b/changelog.d/15323.bugfix
new file mode 100644
index 0000000000..bc1ab35532
--- /dev/null
+++ b/changelog.d/15323.bugfix
@@ -0,0 +1 @@
+Fix a long-standing bug preventing users from joining rooms, that they had been unbanned from, over federation. Contributed by Nico.
diff --git a/synapse/handlers/federation_event.py b/synapse/handlers/federation_event.py
index b7136f8d1c..648843cdbe 100644
--- a/synapse/handlers/federation_event.py
+++ b/synapse/handlers/federation_event.py
@@ -583,7 +583,7 @@ class FederationEventHandler:
 
             await self._check_event_auth(origin, event, context)
             if context.rejected:
-                raise SynapseError(400, "Join event was rejected")
+                raise SynapseError(403, "Join event was rejected")
 
             # the remote server is responsible for sending our join event to the rest
             # of the federation. Indeed, attempting to do so will result in problems
diff --git a/synapse/handlers/room_member.py b/synapse/handlers/room_member.py
index 509c557889..1d8b0aee6f 100644
--- a/synapse/handlers/room_member.py
+++ b/synapse/handlers/room_member.py
@@ -850,64 +850,69 @@ class RoomMemberHandler(metaclass=abc.ABCMeta):
         # `is_partial_state_room` also indicates whether `partial_state_before_join` is
         # partial.
 
-        # TODO: Refactor into dictionary of explicitly allowed transitions
-        # between old and new state, with specific error messages for some
-        # transitions and generic otherwise
-        old_state_id = partial_state_before_join.get(
-            (EventTypes.Member, target.to_string())
-        )
-        if old_state_id:
-            old_state = await self.store.get_event(old_state_id, allow_none=True)
-            old_membership = old_state.content.get("membership") if old_state else None
-            if action == "unban" and old_membership != "ban":
-                raise SynapseError(
-                    403,
-                    "Cannot unban user who was not banned"
-                    " (membership=%s)" % old_membership,
-                    errcode=Codes.BAD_STATE,
-                )
-            if old_membership == "ban" and action not in ["ban", "unban", "leave"]:
-                raise SynapseError(
-                    403,
-                    "Cannot %s user who was banned" % (action,),
-                    errcode=Codes.BAD_STATE,
-                )
-
-            if old_state:
-                same_content = content == old_state.content
-                same_membership = old_membership == effective_membership_state
-                same_sender = requester.user.to_string() == old_state.sender
-                if same_sender and same_membership and same_content:
-                    # duplicate event.
-                    # we know it was persisted, so must have a stream ordering.
-                    assert old_state.internal_metadata.stream_ordering
-                    return (
-                        old_state.event_id,
-                        old_state.internal_metadata.stream_ordering,
-                    )
-
-            if old_membership in ["ban", "leave"] and action == "kick":
-                raise AuthError(403, "The target user is not in the room")
-
-            # we don't allow people to reject invites to the server notice
-            # room, but they can leave it once they are joined.
-            if (
-                old_membership == Membership.INVITE
-                and effective_membership_state == Membership.LEAVE
-            ):
-                is_blocked = await self.store.is_server_notice_room(room_id)
-                if is_blocked:
-                    raise SynapseError(
-                        HTTPStatus.FORBIDDEN,
-                        "You cannot reject this invite",
-                        errcode=Codes.CANNOT_LEAVE_SERVER_NOTICE_ROOM,
-                    )
-        else:
-            if action == "kick":
-                raise AuthError(403, "The target user is not in the room")
-
         is_host_in_room = await self._is_host_in_room(partial_state_before_join)
 
+        # if we are not in the room, we won't have the current state
+        if is_host_in_room:
+            # TODO: Refactor into dictionary of explicitly allowed transitions
+            # between old and new state, with specific error messages for some
+            # transitions and generic otherwise
+            old_state_id = partial_state_before_join.get(
+                (EventTypes.Member, target.to_string())
+            )
+
+            if old_state_id:
+                old_state = await self.store.get_event(old_state_id, allow_none=True)
+                old_membership = (
+                    old_state.content.get("membership") if old_state else None
+                )
+                if action == "unban" and old_membership != "ban":
+                    raise SynapseError(
+                        403,
+                        "Cannot unban user who was not banned"
+                        " (membership=%s)" % old_membership,
+                        errcode=Codes.BAD_STATE,
+                    )
+                if old_membership == "ban" and action not in ["ban", "unban", "leave"]:
+                    raise SynapseError(
+                        403,
+                        "Cannot %s user who was banned" % (action,),
+                        errcode=Codes.BAD_STATE,
+                    )
+
+                if old_state:
+                    same_content = content == old_state.content
+                    same_membership = old_membership == effective_membership_state
+                    same_sender = requester.user.to_string() == old_state.sender
+                    if same_sender and same_membership and same_content:
+                        # duplicate event.
+                        # we know it was persisted, so must have a stream ordering.
+                        assert old_state.internal_metadata.stream_ordering
+                        return (
+                            old_state.event_id,
+                            old_state.internal_metadata.stream_ordering,
+                        )
+
+                if old_membership in ["ban", "leave"] and action == "kick":
+                    raise AuthError(403, "The target user is not in the room")
+
+                # we don't allow people to reject invites to the server notice
+                # room, but they can leave it once they are joined.
+                if (
+                    old_membership == Membership.INVITE
+                    and effective_membership_state == Membership.LEAVE
+                ):
+                    is_blocked = await self.store.is_server_notice_room(room_id)
+                    if is_blocked:
+                        raise SynapseError(
+                            HTTPStatus.FORBIDDEN,
+                            "You cannot reject this invite",
+                            errcode=Codes.CANNOT_LEAVE_SERVER_NOTICE_ROOM,
+                        )
+            else:
+                if action == "kick":
+                    raise AuthError(403, "The target user is not in the room")
+
         if effective_membership_state == Membership.JOIN:
             if requester.is_guest:
                 guest_can_join = await self._can_guest_join(partial_state_before_join)