The following error message is logged by the client protocol module when the routing of a query fails:

2018-09-15 23:09:53   error  : (6) [mariadbclient] Routing the query failed. Session will be closed.

This message should always have an explanatory message logged before giving the reason why the routing failed. If a readwritesplit session loses all connections with the master connecting being the last one, the session still continues. If a session command is executed at this point, the routing would fail with no explanation.