I have found that if an error (like exporting a PDF which generated an error most of the time or hangs), and I previously saved my model BEFORE doing the export, if I am forced to kill the ARIS application task, I have found that those changes are NOT actually saved. When I reopen ARIS application and open that model, it does not reflect my last changes before the lock-up condition.
In the meantime, what I have found is a work-around -- I save the work then I close the model. When I reopen, all my work was saved. Then I try to other activities and if one causes a lock-up, the model is still good.