When you say "it fails", what do mean? accuratly? Do you have a stack trace somewhere or a description.
Otherwise it is hard to say. Which kind of http session replication do you use? (which package? i.e. the jboss one or the jetty one?)
When you are waiting 25-30 seconds (in the "working" scenario, could you do a thread stack dump (press CTRL+BREAK in the console) and send it in a file? We have to try to remove this long wait.