The Kiwi syslog engine rated for approximately 2 million messages per hour. Your ingestion rate is a bit over that... The max messages per hour is also impacted by the number of rules, filters, and actions that have been configured. You can do some tweaks that will get you some additional performance but the real solution is a second syslog server to split the load.
If you watch the Kiwi console there is a queue percentage number at the bottom, you'll likely see that number drop from 100% to 0 very quickly and the server will start dropping messages. You can also log in the error log for additional information about what may have crashed the syslog server. It is located in the application directory.