I was on 6.04 when ot happened, haven't tried since...
I've just gone back to look at it now, it's just the Drafts, Junk, Trash and Chats folders I can't remove from Trash, I can remove all the messages from within them though... here's the error...
Code:
method: FolderActionRequest
msg: cannot modify immutable object: 9175
code: mail.IMMUTABLE_OBJECT
detail: soap:Sender
trace: com.zimbra.cs.mailbox.MailServiceException: cannot modify immutable object: 9175 ExceptionId:btpool0-150://smtp.anheddau.co.uk/service/soap/FolderActionRequest:1281621750928:88779de4f0eaaf51 Code:mail.IMMUTABLE_OBJECT Arg:(itemId, IID, "9175") at com.zimbra.cs.mailbox.MailServiceException.IMMUTABLE_OBJECT(MailServiceException.java:379) at com.zimbra.cs.mailbox.MailItem.delete(MailItem.java:2476) at com.zimbra.cs.mailbox.Folder.deleteSingleFolder(Folder.java:1055) at com.zimbra.cs.mailbox.Folder.delete(Folder.java:1032) at com.zimbra.cs.mailbox.Mailbox.delete(Mailbox.java:5464) at com.zimbra.cs.service.mail.ItemActionHelper.executeLocal(ItemActionHelper.java:352) at com.zimbra.cs.service.mail.ItemActionHelper.schedule(ItemActionHelper.java:314) at com.zimbra.cs.service.mail.ItemActionHelper.HARD_DELETE(ItemActionHelper.java:100) at com.zimbra.cs.service.mail.ItemAction.handleCommon(ItemAction.java:137) at com.zimbra.cs.service.mail.FolderAction.handle(FolderAction.java:108) at com.zimbra.soap.SoapEngine.dispatchRequest(SoapEngine.java:420) at com.zimbra.soap.SoapEngine.dispatch(SoapEngine.java:274) at com.zimbra.soap.SoapEngine.dispatch(SoapEngine.java:158) at com.zimbra.soap.SoapServlet.doWork(SoapServlet.java:291) at com.zimbra.soap.SoapServlet.doPost(SoapServlet.java:212) at javax.servlet.http.HttpServlet.service(HttpServlet.java:727) at com.zimbra.cs.servlet.ZimbraServlet.service(ZimbraServlet.java:181) at javax.servlet.http.HttpServlet.service(HttpServlet.java:820) at org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:511) at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1166) at com.zimbra.cs.servlet.SetHeaderFilter.doFilter(SetHeaderFilter.java:79) at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157) at org.mortbay.servlet.UserAgentFilter.doFilter(UserAgentFilter.java:81) at org.mortbay.servlet.GzipFilter.doFilter(GzipFilter.java:132) at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157) at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:388) at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:216) at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:182) at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:765) at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:418) at org.mortbay.jetty.handler.ContextHandlerCollection.handle(ContextHandlerCollection.java:230) at org.mortbay.jetty.handler.HandlerCollection.handle(HandlerCollection.java:114) at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152) at org.mortbay.jetty.handler.rewrite.RewriteHandler.handle(RewriteHandler.java:230) at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152) at org.mortbay.jetty.handler.DebugHandler.handle(DebugHandler.java:77) at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152) at org.mortbay.jetty.Server.handle(Server.java:326) at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:543) at org.mortbay.jetty.HttpConnection$RequestHandler.content(HttpConnection.java:939) at org.mortbay.jetty.HttpParser.parseNext(HttpParser.java:755) at org.mortbay.jetty.HttpParser.parseAvailable(HttpParser.java:218) at org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:405) at org.mortbay.io.nio.SelectChannelEndPoint.run(SelectChannelEndPoint.java:409) at org.mortbay.thread.BoundedThreadPool$PoolThread.run(BoundedThreadPool.java:451)