OK, I had this error too. Couldn't find the bug, couldn't find the bug, couldn't find the bug. My "tomcat-users" block looked just like this.
<tomcat-users>
<role rolename="manager-gui"/>
<user name="tomcat" password="s3cret" roles="standard,manager-gui"/>
</tomcat-users>
FINALLY FOUND THE BUG. I kept editing the XML inside the XML comment block:
<!--
<tomcat-users>
<role rolename="manager-gui"/>
<user name="tomcat" password="s3cret" roles="standard,manager-gui"/>
</tomcat-users>
-->
DOH!
So: don't forget to remove the "<!--" and "-->".
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