plugin/grails-app/conf/logback-spring.xml

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
    <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
        <encoder class="ch.qos.logback.classic.encoder.PatternLayoutEncoder">
            <charset>UTF-8</charset>
            <pattern>%level %logger - %msg%n</pattern>
        </encoder>
    </appender>

    <root level="error">
        <appender-ref ref="STDOUT" />
    </root>
</configuration>