Element Android currently doesn't send a JSON body for POST `/rooms/{roomId}/receipt/{receiptType}/{eventId}`. It should send a request body, even if that would be the empty body `{}`. This was noticed as part of recent changes in Synapse v1.40.0rc1; a workaround (https://github.com/matrix-org/synapse/pull/10531) will be put into Synapse v1.40.0rc2: The spec is, at the time of writing, wrong (see https://github.com/matrix-org/matrix-doc/issues/3237) but is being fixed by https://github.com/matrix-org/matrix-doc/pull/3238.