CUBRID stores several types of log files. In $CUBRID\log (C:\CUBRID\log for example) you will see 3 folders: broker, manager and server. You will also see error logs for each database and associated cubrid command ran on it (such as createdb, spacedb, lockdb etc.).
On Windows, the broker files can be found in:
On Linux, the path is $CUBRID/log/broker/sql_log and error_log respectively.
The sql_log folder contains all the queries executed, while the error_log contains all the errors that have been thrown by CUBRID while running the queries.
You will also see a query_editor.access and/or a broker.access file in $CUBRID\log\broker. These files hold the IP address and date for each access to a database.
Debugging can be easily done by looking at the log files. In error_log folder you will see all commands that have failed to execute due to constraint violations, non-existent tables, syntax errors and so forth. In sql_log you can see each command ran by each broker and query editor. The commands are given in prepared SQL format followed by the bind statements.
Note: In sql_log folder you can find several slow.log files which contain all the queries that have taken longer than 1 second to execute. This is an important debugging tool as you will be able to see all the slow queries that have been executed.
The $CUBRID\log\manager folder holds information regarding the execution of the cubrid manager service, such as connection information and executed commands.
The $CUBRID\log\server folder holds the logs for each running database server and each file inside this folder is named according to when the database was first started.
In CUBRID Manager, you will see a tab Logs that contains Broker/Manager/Server links. The Broker tab divides into Access log, Error log and Admin log. The Manager tab has an access log and an error log. The Server tab holds each database name and for each database you can see the log file for its entire running time.

For more specific information visit CUBRID Log Files.