Clearing eclipse cache for "referenced file contains error" problems

I use castor for xml binding/generation and this library works with mapping files.

The mapping files have the standart DOCTYPE header for castor.

<!DOCTYPE mapping PUBLIC "-//EXOLAB/Castor Mapping DTD Version 1.0//EN"    "http://castor.org/mapping.dtd"> 

In my eclipse installations, I get the following error

Referenced file contains errors (http://castor.org/mapping.dtd)

What I tried (and did not work):

• Clear network cache in eclipse Prefs-General-Network Connections-Cache
• Changed eclipse version, using eclipse 3.8.2 now; same problem in both versions.
• Run eclipse with -clean option
• Copied the project to a new workspace

Any suggestions for an alternative hard reset?

The castor docs clearly states the usage of schema files here http://castor.codehaus.org/schema.html and it clearly states

Mapping DTD -//EXOLAB/Castor Object Mapping DTD Version 1.0//EN http://castor.org/mapping.dtd

But when you click on the castor.org/mapping.dtd link on the schema url, it links to http://castor.codehaus.org/mapping.dtd

So, in fact, the correct dtd location is http://castor.codehaus.org/mapping.dtd , not castor.org/mapping.dtd . This is clearly an ambiguity, but solves the "referenced file contains error" problem