Experienced the following problem:
Selected the lvl.30 character and played through the Invaded mission once.
After this joined to a player from the Backup request map screen. The player was at the end of the mission, but I spawned outside at the Mission start "panel".
To reaching the other player I need to run through the whole building, but after a while the path ended up with a locked door. This door can be opening if playing normally, but this time was closed and without the "Open" option. The mission marker was on the door, and saw a Restock box behind.