we have just uploaded the 8.6.4 minor release to our download server.
You can find an overview of changes at https://documentation.kopano.io/kopano_changelog/kc.html#kopano-core-8-6-4-8-6-4-0
Please be aware that due to a build failure this release currently does not include packages for Ubuntu 14.04. Packages for 14.04 will be added once the build succeeds again.
In general we recommend users of Ubuntu 14.04 to switch to a newer Ubuntu release, since 8.6 will be the last Kopano release to include packages for this distribution and with packages for Ubuntu 16.04 and 18.04 appropriate alternatives exist.