Belated thanks, Padmanabhan. Could be useful to anyone for whom increasing numprocs doesn't work (though I don't really understand why not).
Richard
I just got the same error myself, where increasing numprocs didn't work. It turned out that the task wasn't actually running out of processes or threads at all; rather, it was running out of memory while creating new worker threads. So the error message was rather a red herring. Just increasing the memory limit solved it.