I am able to backup individual users accounts with:
zmbackup -f -a
admin@domain.com
The accounts.xml file looks ok too.
Here is the output from a zmbackup -f -d (debug):
zimbra@zimbra:~/redolog$ zmbackup -f -d
Error occurred: invalid request: Missing account list
class=com.zimbra.common.soap.SoapFaultException
message=invalid request: Missing account list
mIsReceiversFault=false
mIsLocal=false
mDetail=<soap

etail><Error xmlns="urn:zimbra"><Code>service.INVALID_REQUEST</Code><Trace>com.zimbra.common.service.ServiceExcep tion: invalid request: Missing account lis t
ExceptionId:btpool0-356://localhost:7071/service/admin/soap/BackupRequest:1301597638074:ddd07dcd24636c8c
Code:service.INVALID_REQUEST
at com.zimbra.common.service.ServiceException.INVALID _REQUEST(ServiceException.java:255)
at com.zimbra.cs.service.backup.Backup.handleNetworkR equest(Backup.java:143)
at com.zimbra.cs.service.NetworkDocumentHandler.handl e(NetworkDocumentHandler.java:22)
at com.zimbra.soap.SoapEngine.dispatchRequest(SoapEng ine.java:420)
at com.zimbra.soap.SoapEngine.dispatch(SoapEngine.jav a:274)
at com.zimbra.soap.SoapEngine.dispatch(SoapEngine.jav a:158)
at com.zimbra.soap.SoapServlet.doWork(SoapServlet.jav a:291)
at com.zimbra.soap.SoapServlet.doPost(SoapServlet.jav a:212)
at javax.servlet.http.HttpServlet.service(HttpServlet .java:727)
at com.zimbra.cs.servlet.ZimbraServlet.service(Zimbra Servlet.java:181)
at javax.servlet.http.HttpServlet.service(HttpServlet .java:820)
at org.mortbay.jetty.servlet.ServletHolder.handle(Ser vletHolder.java:511)
at org.mortbay.jetty.servlet.ServletHandler$CachedCha in.doFilter(ServletHandler.java:1166)
at com.zimbra.cs.servlet.SetHeaderFilter.doFilter(Set HeaderFilter.java:79)
at org.mortbay.jetty.servlet.ServletHandler$CachedCha in.doFilter(ServletHandler.java:1157)
at org.mortbay.servlet.UserAgentFilter.doFilter(UserA gentFilter.java:81)
at org.mortbay.servlet.GzipFilter.doFilter(GzipFilter .java:155)
at org.mortbay.jetty.servlet.ServletHandler$CachedCha in.doFilter(ServletHandler.java:1157)
at org.mortbay.jetty.servlet.ServletHandler.handle(Se rvletHandler.java:388)
at org.mortbay.jetty.security.SecurityHandler.handle( SecurityHandler.java:216)
at org.mortbay.jetty.servlet.SessionHandler.handle(Se ssionHandler.java:182)
at org.mortbay.jetty.handler.ContextHandler.handle(Co ntextHandler.java:765)
at org.mortbay.jetty.webapp.WebAppContext.handle(WebA ppContext.java:418)
at org.mortbay.jetty.handler.ContextHandlerCollection .handle(ContextHandlerCollection.java:230)
at org.mortbay.jetty.handler.HandlerCollection.handle (HandlerCollection.java:114)
at org.mortbay.jetty.handler.HandlerWrapper.handle(Ha ndlerWrapper.java:152)
at org.mortbay.jetty.handler.rewrite.RewriteHandler.h andle(RewriteHandler.java:230)
at org.mortbay.jetty.handler.HandlerWrapper.handle(Ha ndlerWrapper.java:152)
at org.mortbay.jetty.handler.DebugHandler.handle(Debu gHandler.java:77)
at org.mortbay.jetty.handler.HandlerWrapper.handle(Ha ndlerWrapper.java:152)
at org.mortbay.jetty.Server.handle(Server.java:326)
at org.mortbay.jetty.HttpConnection.handleRequest(Htt pConnection.java:543)
at org.mortbay.jetty.HttpConnection$RequestHandler.co ntent(HttpConnection.java:939)
at org.mortbay.jetty.HttpParser.parseNext(HttpParser. java:755)
at org.mortbay.jetty.HttpParser.parseAvailable(HttpPa rser.java:218)
at org.mortbay.jetty.HttpConnection.handle(HttpConnec tion.java:405)
at org.mortbay.io.nio.SelectChannelEndPoint.run(Selec tChannelEndPoint.java:409)
at org.mortbay.thread.BoundedThreadPool$PoolThread.ru n(BoundedThreadPool.java:451)
</Trace></Error></soap

etail>
mFault=<soap:Fault><soap:Code><soap:Value>soap:Sen der</soap:Value></soap:Code><soap:Reason><soap:Text>invalid request: Missing account list</soap:Text></soap:Reason><soap

etail><Error xmlns="urn:zimbra"><Code>service.INVALID_REQUEST</Code><Trace>com.zimbra.common.service.ServiceExcep tion: invalid request: Missing account list
ExceptionId:btpool0-356://localhost:7071/service/admin/soap/BackupRequest:1301597638074:ddd07dcd24636c8c
Code:service.INVALID_REQUEST
at com.zimbra.common.service.ServiceException.INVALID _REQUEST(ServiceException.java:255)
at com.zimbra.cs.service.backup.Backup.handleNetworkR equest(Backup.java:143)
at com.zimbra.cs.service.NetworkDocumentHandler.handl e(NetworkDocumentHandler.java:22)
at com.zimbra.soap.SoapEngine.dispatchRequest(SoapEng ine.java:420)
at com.zimbra.soap.SoapEngine.dispatch(SoapEngine.jav a:274)
at com.zimbra.soap.SoapEngine.dispatch(SoapEngine.jav a:158)
at com.zimbra.soap.SoapServlet.doWork(SoapServlet.jav a:291)
at com.zimbra.soap.SoapServlet.doPost(SoapServlet.jav a:212)
at javax.servlet.http.HttpServlet.service(HttpServlet .java:727)
at com.zimbra.cs.servlet.ZimbraServlet.service(Zimbra Servlet.java:181)
at javax.servlet.http.HttpServlet.service(HttpServlet .java:820)
at org.mortbay.jetty.servlet.ServletHolder.handle(Ser vletHolder.java:511)
at org.mortbay.jetty.servlet.ServletHandler$CachedCha in.doFilter(ServletHandler.java:1166)
at com.zimbra.cs.servlet.SetHeaderFilter.doFilter(Set HeaderFilter.java:79)
at org.mortbay.jetty.servlet.ServletHandler$CachedCha in.doFilter(ServletHandler.java:1157)
at org.mortbay.servlet.UserAgentFilter.doFilter(UserA gentFilter.java:81)
at org.mortbay.servlet.GzipFilter.doFilter(GzipFilter .java:155)
at org.mortbay.jetty.servlet.ServletHandler$CachedCha in.doFilter(ServletHandler.java:1157)
at org.mortbay.jetty.servlet.ServletHandler.handle(Se rvletHandler.java:388)
at org.mortbay.jetty.security.SecurityHandler.handle( SecurityHandler.java:216)
at org.mortbay.jetty.servlet.SessionHandler.handle(Se ssionHandler.java:182)
at org.mortbay.jetty.handler.ContextHandler.handle(Co ntextHandler.java:765)
at org.mortbay.jetty.webapp.WebAppContext.handle(WebA ppContext.java:418)
at org.mortbay.jetty.handler.ContextHandlerCollection .handle(ContextHandlerCollection.java:230)
at org.mortbay.jetty.handler.HandlerCollection.handle (HandlerCollection.java:114)
at org.mortbay.jetty.handler.HandlerWrapper.handle(Ha ndlerWrapper.java:152)
at org.mortbay.jetty.handler.rewrite.RewriteHandler.h andle(RewriteHandler.java:230)
at org.mortbay.jetty.handler.HandlerWrapper.handle(Ha ndlerWrapper.java:152)
at org.mortbay.jetty.handler.DebugHandler.handle(Debu gHandler.java:77)
at org.mortbay.jetty.handler.HandlerWrapper.handle(Ha ndlerWrapper.java:152)
at org.mortbay.jetty.Server.handle(Server.java:326)
at org.mortbay.jetty.HttpConnection.handleRequest(Htt pConnection.java:543)
at org.mortbay.jetty.HttpConnection$RequestHandler.co ntent(HttpConnection.java:939)
at org.mortbay.jetty.HttpParser.parseNext(HttpParser. java:755)
at org.mortbay.jetty.HttpParser.parseAvailable(HttpPa rser.java:218)
at org.mortbay.jetty.HttpConnection.handle(HttpConnec tion.java:405)
at org.mortbay.io.nio.SelectChannelEndPoint.run(Selec tChannelEndPoint.java:409)
at org.mortbay.thread.BoundedThreadPool$PoolThread.ru n(BoundedThreadPool.java:451)
</Trace></Error></soap

etail></soap:Fault>