Eclipse reduces it's log monitoring to following stdout. This is not at all helpful for debugging a webapp like JForum, as template errors will not show. In a browser, template errors look like this,
An error has occurred.
For detailed error information, please see the HTML source code, and contact the forum Administrator.
When launching through Eclipse, the Tomcat std out is in the Eclipse console view. And yes, template errors do show up there.
Not showing details of errors in the browser is actually a security feature. The idea is not to give hackers to much information about the implementation of the code and how close they are getting to an attack.