What does this error mean: PWC1635: Illegal access: this web application instance has been stopped already?

The complete error message reads as follows:

PWC1635: Illegal access: this web application instance has been stopped already (the eventual following stack trace is caused by an error thrown for debugging purposes as well as to attempt to terminate the thread which caused the illegal access, and has no functional impact)

This means that a component is asking a web application's classloader to load a resource before the webapp has been started, or after it has been stopped.

In the latter case, this would happen if the web application's classloader was cached, but not cleared from the cache when the web application was stopped or undeployed. For example, this error is seen if an application calls Thread.setContextClassLoader(WebappClassLoader), and forgets to unset it when the associated web application is stopped or undeployed. Next time the thread is re-used, and an attempt is made to use its context classloader to load a resource, the above error will be reported.