occasional NoClassDefFoundError

[ahoward333]

One of our deployment occasionally gets a NoClassDefFoundError.
We have deployed with the PermGen garbage collection turned on, and I'm curious to know if that is perhaps resulting in an error like this:
java.lang.NoClassDefFoundError: com/opensymphony/module/sitemesh/util/FastByteArrayOutputStream
com.opensymphony.module.sitemesh.filter.Buffer.getOutputStream(Buffer.java:68)
com.opensymphony.module.sitemesh.filter.PageResponseWrapper$4.create(PageResponseWrapper.java:84)
...

Note: It's not always a SiteMesh library. It seems to be a different library each time, sometimes even lower-level JBoss libs.

I'm totally lost here, as identical installations elsewhere have NEVER seen an error like this, so I'm not sure where to begin.
Is it possible that the class unloading i've enabled:
-XX:+UseConcMarkSweepGC -XX:+CMSPermGenSweepingEnabled -XX:+CMSClassUnloadingEnabled
is interfering?
Could this be due to a network admin stopping and/or starting JBoss improperly (GUI 'X' followed by 'End Now').

Any help would be greatly appreciated!
-Trey

[ahoward333]

Well, we found the cause, so I'm posting here in case anyone should encounter this problem:

The network admin had installed, unbeknownst to us, an antivirus/firewall app that was blocking ports required by JBoss. He didn't notice the server log was full of errors indicating that ports were in use, etc.