That is no bug in fact, the server needs to load the data of any item in your bag, which will require some time. (The more time the more items, usually.)
Be patient and keep waiting, as it can take up to 5 minutes in worst cases, but the server itself won't go down, nor will your client.
Besides, that is supposed to happen like once a day, probably due to entering many new areas in a row.
Anyways - waiting should fix your problem.
Enjoy your game!