I have been having teh same problems for some time now. When I assign a task for my student and sheer it with them, a few students are able to do the task with no problems but others, when they open it, they get the message that the task is not available anymore.
Those tasks are usually linked to a timed exercise, so I wonder if it has to do with the time limit. Perhaps when they access it they have run out of that time? Perhaps if the open the task once the are not allowed to open it again??