We will be releasing a Nagios Core 3.4.1 release shortly to fix a bug that was found in the recent 3.4.0 release. Until the new release is available, we would advise that anyone running the 3.4.0 code rollback to 3.3.1. You can download 3.3.1 on the downloads page.
Nagios Update: XI 5.6.6
Nagios XI update 5.6.6 is now released! This update resolves a number of minor bugs and one security vulnerability fix. This version also includes improvements