Most likely because either yourself or one of your team mates has been disconnected from the server which unfortunately means for you is a whole crash. Thread moved.
It can happen so you must be aware that trading / sending mail at this stage of the open stress test if risky and un-refundable. Problem resolved and moved.
It might or might not bug out and there's nothing that can be done about it at this stage as the Developers are hard at work getting it sorted, anyway moved.
It sounds like that this is down to lag or a severe drop in FPS. Anyway as this thread has been around for over a week and OP has not re-visited it i'm just going to move it to Resolved. If you wish the topic to be moved back here send me a message.
It's known my friend and at this stage of the stress test it is advised that you do not expect all your trades / mails to be completely without problems. Moved.