Given that its a ClassNotFoundException, I would guess that its a classloading problem where bvy Xwork can nolonger see the plugin action classes. Full stack trace follows.

com.opensymphony.xwork.XworkException: Unable to instantiate Action, com.adaptavist.confluence.sitebuilder.ConfigureThemeAction, defined for 'configuretheme' in namespace '/themes/sitebuilder'com.adaptavist.confluence.sitebuilder.ConfigureThemeAction
with nested exception
java.lang.ClassNotFoundException: com.adaptavist.confluence.sitebuilder.ConfigureThemeAction
at com.opensymphony.xwork.DefaultActionInvocation.createAction(DefaultActionInvocation.java:220)
at com.opensymphony.xwork.DefaultActionInvocation.init(DefaultActionInvocation.java:272)
at com.opensymphony.xwork.DefaultActionInvocation.(DefaultActionInvocation.java:65)
at com.opensymphony.xwork.DefaultActionInvocation.(DefaultActionInvocation.java:58)
at com.opensymphony.xwork.DefaultActionProxyFactory.createActionInvocation(DefaultActionProxyFactory.java:32)
at com.opensymphony.xwork.DefaultActionProxy.prepare(DefaultActionProxy.java:124)
at com.opensymphony.xwork.DefaultActionProxy.(DefaultActionProxy.java:75)
at com.opensymphony.xwork.DefaultActionProxyFactory.createActionProxy(DefaultActionProxyFactory.java:45)
at com.opensymphony.webwork.dispatcher.ServletDispatcher.serviceAction(ServletDispatcher.java:227)
at com.opensymphony.webwork.dispatcher.ServletDispatcher.service(ServletDispatcher.java:199)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:92)
at com.caucho.server.dispatch.ServletFilterChain.doFilter(ServletFilterChain.java:99)
at com.opensymphony.module.sitemesh.filter.PageFilter.parsePage(PageFilter.java:118)
at com.atlassian.confluence.util.profiling.ProfilingPageFilter.parsePage(ProfilingPageFilter.java:143)
at com.opensymphony.module.sitemesh.filter.PageFilter.doFilter(PageFilter.java:52)
at com.caucho.server.dispatch.FilterFilterChain.doFilter(FilterFilterChain.java:70)
at com.atlassian.confluence.util.ServletContextThreadLocalFilter.doFilter(ServletContextThreadLocalFilter.java:30)
at com.caucho.server.dispatch.FilterFilterChain.doFilter(FilterFilterChain.java:70)
at com.atlassian.confluence.util.UserThreadLocalFilter.doFilter(UserThreadLocalFilter.java:45)
at com.caucho.server.dispatch.FilterFilterChain.doFilter(FilterFilterChain.java:70)
at com.atlassian.seraph.filter.SecurityFilter.doFilter(SecurityFilter.java:182)
at com.caucho.server.dispatch.FilterFilterChain.doFilter(FilterFilterChain.java:70)
at com.atlassian.seraph.filter.LoginFilter.doFilter(LoginFilter.java:177)
at com.caucho.server.dispatch.FilterFilterChain.doFilter(FilterFilterChain.java:70)
at com.atlassian.johnson.filters.JohnsonFilter.doFilter(JohnsonFilter.java:96)
at com.caucho.server.dispatch.FilterFilterChain.doFilter(FilterFilterChain.java:70)
at org.springframework.orm.hibernate.support.OpenSessionInViewFilter.doFilterInternal(OpenSessionInViewFilter.java:170)
at bucket.custom.FlushingSpringSessionInViewFilter.doFilterInternal(FlushingSpringSessionInViewFilter.java:31)
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:73)
at com.caucho.server.dispatch.FilterFilterChain.doFilter(FilterFilterChain.java:70)
at com.atlassian.util.profiling.filters.ProfilingFilter.doFilter(ProfilingFilter.java:122)
at com.atlassian.confluence.util.error.ProfilingAndErrorFilter.doFilter(ProfilingAndErrorFilter.java:27)
at com.caucho.server.dispatch.FilterFilterChain.doFilter(FilterFilterChain.java:70)
at com.atlassian.core.filters.gzip.GzipFilter.doFilter(GzipFilter.java:53)
at com.caucho.server.dispatch.FilterFilterChain.doFilter(FilterFilterChain.java:70)
at com.atlassian.core.filters.AbstractEncodingFilter.doFilter(AbstractEncodingFilter.java:37)
at com.caucho.server.dispatch.FilterFilterChain.doFilter(FilterFilterChain.java:70)
at com.caucho.server.webapp.WebAppFilterChain.doFilter(WebAppFilterChain.java:163)
at com.caucho.server.dispatch.ServletInvocation.service(ServletInvocation.java:208)
at com.caucho.server.http.HttpRequest.handleRequest(HttpRequest.java:259)
at com.caucho.server.port.TcpConnection.run(TcpConnection.java:363)
at com.caucho.util.ThreadPool.runTasks(ThreadPool.java:490)
at com.caucho.util.ThreadPool.run(ThreadPool.java:423)
at java.lang.Thread.run(Thread.java:595)
with nested exception java.lang.ClassNotFoundException: com.adaptavist.confluence.sitebuilder.ConfigureThemeAction
java.lang.ClassNotFoundException: com.adaptavist.confluence.sitebuilder.ConfigureThemeAction
at com.atlassian.confluence.plugin.ConfluencePluginObjectFactory.loadClassFromPlugins(ConfluencePluginObjectFactory.java:66)
at com.atlassian.confluence.plugin.ConfluencePluginObjectFactory.getClassInstance(ConfluencePluginObjectFactory.java:35)
at com.opensymphony.xwork.ObjectFactory.buildBean(ObjectFactory.java:95)
at com.opensymphony.xwork.ObjectFactory.buildAction(ObjectFactory.java:77)
at com.opensymphony.xwork.DefaultActionInvocation.createAction(DefaultActionInvocation.java:199)
at com.opensymphony.xwork.DefaultActionInvocation.init(DefaultActionInvocation.java:272)
at com.opensymphony.xwork.DefaultActionInvocation.(DefaultActionInvocation.java:65)
at com.opensymphony.xwork.DefaultActionInvocation.(DefaultActionInvocation.java:58)
at com.opensymphony.xwork.DefaultActionProxyFactory.createActionInvocation(DefaultActionProxyFactory.java:32)
at com.opensymphony.xwork.DefaultActionProxy.prepare(DefaultActionProxy.java:124)
at com.opensymphony.xwork.DefaultActionProxy.(DefaultActionProxy.java:75)
at com.opensymphony.xwork.DefaultActionProxyFactory.createActionProxy(DefaultActionProxyFactory.java:45)
at com.opensymphony.webwork.dispatcher.ServletDispatcher.serviceAction(ServletDispatcher.java:227)
at com.opensymphony.webwork.dispatcher.ServletDispatcher.service(ServletDispatcher.java:199)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:92)
at com.caucho.server.dispatch.ServletFilterChain.doFilter(ServletFilterChain.java:99)
at com.opensymphony.module.sitemesh.filter.PageFilter.parsePage(PageFilter.java:118)
at com.atlassian.confluence.util.profiling.ProfilingPageFilter.parsePage(ProfilingPageFilter.java:143)
at com.opensymphony.module.sitemesh.filter.PageFilter.doFilter(PageFilter.java:52)
at com.caucho.server.dispatch.FilterFilterChain.doFilter(FilterFilterChain.java:70)
at com.atlassian.confluence.util.ServletContextThreadLocalFilter.doFilter(ServletContextThreadLocalFilter.java:30)
at com.caucho.server.dispatch.FilterFilterChain.doFilter(FilterFilterChain.java:70)
at com.atlassian.confluence.util.UserThreadLocalFilter.doFilter(UserThreadLocalFilter.java:45)
at com.caucho.server.dispatch.FilterFilterChain.doFilter(FilterFilterChain.java:70)
at com.atlassian.seraph.filter.SecurityFilter.doFilter(SecurityFilter.java:182)
at com.caucho.server.dispatch.FilterFilterChain.doFilter(FilterFilterChain.java:70)
at com.atlassian.seraph.filter.LoginFilter.doFilter(LoginFilter.java:177)
at com.caucho.server.dispatch.FilterFilterChain.doFilter(FilterFilterChain.java:70)
at com.atlassian.johnson.filters.JohnsonFilter.doFilter(JohnsonFilter.java:96)
at com.caucho.server.dispatch.FilterFilterChain.doFilter(FilterFilterChain.java:70)
at org.springframework.orm.hibernate.support.OpenSessionInViewFilter.doFilterInternal(OpenSessionInViewFilter.java:170)
at bucket.custom.FlushingSpringSessionInViewFilter.doFilterInternal(FlushingSpringSessionInViewFilter.java:31)
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:73)
at com.caucho.server.dispatch.FilterFilterChain.doFilter(FilterFilterChain.java:70)
at com.atlassian.util.profiling.filters.ProfilingFilter.doFilter(ProfilingFilter.java:122)
at com.atlassian.confluence.util.error.ProfilingAndErrorFilter.doFilter(ProfilingAndErrorFilter.java:27)
at com.caucho.server.dispatch.FilterFilterChain.doFilter(FilterFilterChain.java:70)
at com.atlassian.core.filters.gzip.GzipFilter.doFilter(GzipFilter.java:53)
at com.caucho.server.dispatch.FilterFilterChain.doFilter(FilterFilterChain.java:70)
at com.atlassian.core.filters.AbstractEncodingFilter.doFilter(AbstractEncodingFilter.java:37)
at com.caucho.server.dispatch.FilterFilterChain.doFilter(FilterFilterChain.java:70)
at com.caucho.server.webapp.WebAppFilterChain.doFilter(WebAppFilterChain.java:163)
at com.caucho.server.dispatch.ServletInvocation.service(ServletInvocation.java:208)
at com.caucho.server.http.HttpRequest.handleRequest(HttpRequest.java:259)
at com.caucho.server.port.TcpConnection.run(TcpConnection.java:363)
at com.caucho.util.ThreadPool.runTasks(ThreadPool.java:490)
at com.caucho.util.ThreadPool.run(ThreadPool.java:423)
at java.lang.Thread.run(Thread.java:595)