Cannot open build.xml in Eclipse

If you open any of your ant build scipt files (build.xml, build.properties.xml, etc)
and you get the following class cast exception:
org.apache.xerces.parsers.XIncludeAwareParserConfiguration cannot be cast to
org.apache.xerces.xni.parser.XMLParserConfiguration

You may solve this trying two ways:

1) Close and open Eclipse (this solves this issue in the majority of cases)

2) Reset Ant option (Window-Preferences-Ant-Restore Defaults) the close and open Eclipse again