Tomcat won't display current error messages in the log files.
This error is common in Tomcat 5.5 running in Linux.
This problem will prevent Tomcat from generating other error messages beside the following:
"ERROR org.apache.commons.digester.Digester - Parse Fatal Error at line 24 column 7: The string "--" is not permitted within comments.
org.xml.sax.SAXParseException: The string "--" is not permitted within comments. "
What is causing this issue is a file called Admin.xml file located at .../tomcat/conf/catalina/localhost directory. This file has the string "--" inside a commented line.
To correct this issue just replace the Admin.xml file with the one found in this KB article.