Invite plugin throws IncompatibleClassChangeError exception

System Information System Date Tuesday, 21 Aug 2007 System Time 08:54:47 System Favourite Colour Myrtle Java Version 1.5.0_08 Java Vendor Sun Microsystems Inc. JVM Version 1.0 JVM Vendor Sun Microsystems Inc. JVM Implementation Version 1.5.0_08-b03 Java Runtime Java(TM) 2 Runtime Environment, Standard Edition Java VM Java HotSpot(TM) Client VM User Name ganand User Timezone Australia/Sydney Operating System Windows XP 5.1 OS Architecture x86 Filesystem Encoding Cp1252 Java VM Memory Statistics Total Memory 127 MB Free Memory 20 MB Used Memory 107 MB Memory Graph [Used Memory (86 %)] [Free Memory (14 %)] 14 % Free Runtime Information Database Dialect net.sf.hibernate.dialect.HSQLDialect Database Connection URL jdbc:hsqldb:C:\data\standalones\confluence-2.5.6-std\data/database/confluencedb Database Driver Name org.hsqldb.jdbcDriver Database Driver Version 1.8 Application Server Apache Tomcat/5.5.15 Servlet Version 2.4 Confluence Home C:\data\standalones\confluence-2.5.6-std\data Uptime 39 minutes, 49 seconds Confluence Version 2.5.6 Build Number 812