This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
There are sevaral forums discussing the topic. The reason to that is that the object on the server has been garbage collected like stated in the forums. To avoid this you have to hold a reference to the object. Details you can find e.g. at sun's forum, Stackoverflow or here.