I have encountered this bug twice now, my comp specs are more than sufficient and flash is fully updated. I am using Firefox.
Whenever I start a mission / instance, everything loads fine however the flash gets "frozen", but not entirely because when I click it moves my leaders nametag (only) around, numbers from zombies being shot pop up still but everything animation wise is stuck. The first time it happened I refreshed the page and it was like I never ran the mission, did it again and it was fine. But the second time now when I encountered the bug on Riverside Hospital, I refreshed, it gave me my people back like I never ran it but gave me the mission lockout like I did. =(