Just upgrade confluence to 5.7.1. From the log message, got the following warning message:

Mar 12, 2015 2:09:08 PM org.apache.tomcat.util.digester.SetPropertiesRule begin
WARNING: [SetPropertiesRule]

{Server}

Setting property 'debug' to '0' did not find a matching property.
Mar 12, 2015 2:09:08 PM org.apache.catalina.startup.SetAllPropertiesRule begin
WARNING: [SetAllPropertiesRule]

{Server/Service/Connector} Setting property 'minProcessors' to '5' did not find a matching property.
Mar 12, 2015 2:09:08 PM org.apache.catalina.startup.SetAllPropertiesRule begin
WARNING: [SetAllPropertiesRule]{Server/Service/Connector}

Setting property 'maxProcessors' to '75' did not find a matching property.
Mar 12, 2015 2:09:08 PM org.apache.catalina.startup.SetAllPropertiesRule begin
WARNING: [SetAllPropertiesRule]

{Server/Service/Connector} Setting property 'debug' to '0' did not find a matching property.
Mar 12, 2015 2:09:08 PM org.apache.catalina.startup.SetAllPropertiesRule begin
WARNING: [SetAllPropertiesRule]{Server/Service/Connector}

Setting property 'useURIValidationHack' to 'false' did not find a matching property.
Mar 12, 2015 2:09:08 PM org.apache.tomcat.util.digester.SetPropertiesRule begin
WARNING: [SetPropertiesRule]

{Server/Service/Engine}

Setting property 'debug' to '0' did not find a matching property.
Mar 12, 2015 2:09:08 PM org.apache.tomcat.util.digester.SetPropertiesRule begin
WARNING: [SetPropertiesRule]

{Server/Service/Engine/Host}

Setting property 'debug' to '0' did not find a matching property.
Mar 12, 2015 2:09:08 PM org.apache.tomcat.util.digester.SetPropertiesRule begin
WARNING: [SetPropertiesRule]

{Server/Service/Engine/Host/Context}

Setting property 'debug' to '0' did not find a matching property.