Notifier may leave a lock behind
If the notifier doesn't clean up its lock for whatever reason, more notifiers flood the router, waiting for the lock to get released. Try to find out why it happens. Try to think of some way to mitigate the problem (giving up waiting after n minutes, or whatever).
A log is in https://tracker.nic.cz/Ticket/Display.html?id=675005, some more info in https://www.turris.cz/forum/topic_show.pl?tid=1220.