So, installing any plugin (like org.apache.xerces required by ADT) that required javax.xml caused two instances of those package to live together in different classloaders. Whenever they met, CCE or CNFE was thrown. Typical scenario was:
- Plugin A is started and loads javax.xml (VM version)
- Plugin B is started and loads javax.xml (bundle version).
- Plugin C calls A, gets some javax.xml class instance, and passes it to plugin B. Since each plugin has a separate classloader, CCE is thrown.
Unfortunately there is no way to fix the Eclipse package itself - as it is not broken. It is just the matter of integration - and it will not happen until there is someone who is willing to package ADT for Fedora. I'm going to propose this task as a GSoC project (unless someone steps up earlier).