Unknown Issue

Look at the following logs:

Apache Tomcat Logs (/usr/share/tomcat/default/logs)  
Can be useful in cases of application server failures. In cases of network problems or Java problems:

• access_tomcat.<date>.log.*
• catalina.<date>.log.*
• localhost.<date>.log.*
• worker.log.*

Nginx Logs (/var/log/nginx)

Database Logs (typically it is /var/lib/pgsql/<version number>/data/).  If the service is running you can use the following command to get the exact path: 

ps -ef | grep postgres 

Memory and Space issues: Check the output of the following commands:

• free -g
• top -cH