During the upgrade to PuppetDB 2.0, PuppetDB failed to start. The location of the config file for the logging-config changed.
The original location is in the link below.
/etc/puppetdb/log4j.properties
The /etc/puppetdb/conf.d/config.ini file should be updated to point to the link below.
logging-config = /etc/puppetdb/logback.xml