Why is it so hard to find error entries in the server log? Just open the log with a text editor and tell it to find "ERROR" as a case-sensitive whole word. (If your text editor doesn't do that, get a better text editor!)
JBoss AS versions 4.x and 5.x used Log4J for logging. Look for the server/xxx/cong/jboss-og4j.xml (or just log4j.xml in earlier versions). To limit server log output to ERROR, add a threshold line to the FILE appender, as shown below (this text is from 5.1.0, other versions migth differ slightly in their contents):
<appender name="FILE" class="org.jboss.logging.appender.DailyRollingFileAppender">
<errorHandler class="org.jboss.logging.util.OnlyOnceErrorHandler"/>
<param name="File" value="${jboss.server.log.dir}/server.log"/>
<param name="Append" value="true"/>
<param name="Threshold" value="ERROR"/> <!-- add this line -->
. . .
</appender>