The cogserver.log file is a good source of information when troubleshooting. But default the cogserver.log file is a 10 MB file (set in Cognos Configuration Manager) in xml format and hard to read.

127.0.0.1:9300 3048 2009-06-04 09:16:53.218 +1     stopper-logService LOGSV 2015 1 server.Audit.IPF StopService LogService  Success  <parameters><item name="Port"><![CDATA[9362]]></item><item name="Mode"><![CDATA[TCP]]></item><item name="Secure"><![CDATA[FALSE]]></item></parameters>
127.0.0.1:9300 1880 2009-06-05 10:07:53.187 +1     main LOGSV 2015 1 server.Audit.IPF StartService LogService  FAILED Log Server Database error <parameters><item name="DB URL"><![CDATA[jdbc:JSQLConnect://localhost:1433/Audit]]></item><item name="DB Error"><![CDATA[Failed Logon:com.jnetdirect.jsql.JSQLException: TCP/IP connection failed to host:localhost java.net.ConnectException: Connection refused: connect url:jdbc:JSQLConnect://localhost:1433/Audit]]></item></parameters>
127.0.0.1:9300 1880 2009-06-05 10:07:53.234 +1     main LOGSV 2015 1 server.Audit.IPF StartService LogService  Success  <parameters><item name="Port"><![CDATA[9362]]></item><item name="Mode"><![CDATA[TCP]]></item><item name="Secure"><![CDATA[FALSE]]></item></parameters> 

Since Cognos 8.1 there is a handy tool called LogviewV2.exe and can be found in the c8\bin directory. Start LogviewV2.exe and load the cogserver.log file. Now all the messages and errors are formatted, can be read much better.

If you use the summarize button in the toolbar you get a summary of all the errors in the cogserver.log file. Use this file to solve issues which are occurring frequently