The Event Console was not deleting outdated entries from the event history. With the default settings it is intended to delete entries older than 365 days from the EC archive. This did not work.
A message like this can be found each "Housekeeping interval", normaly 1 minute:
[1509618281.352829] Error expiring log files: year out of range
This time window can be configured with the setting "Event history lifetime". As a workaround you can simply open this setting and save it with the default value. This will make the event deletion work as expected.
When you are affected, this may result in a way too huge Event Console archive directory (~/var/mkeventd/history) which may result in slow "Event Console History" views (depends on your filtering). You could clean up the archive directory by hand to improve the situation.
After applying the update, the next housekeeping run will clean up all your old archived events.