NPCs stop respawning

Game mode: Private Testlive

My problem would be that me with my friends rented a server for us and after roughly one day of gameplay NPCs stopped spawning on the Pirate Ship. We thought that it was some kind of bug and than did a server restart. After that all NPCs are disappeared and even if i go back to older save games, like when we started the server for the first time, there are still no NPCs in that savefile, My question would be that if similar thing haapend to anyone before on any server, and if sombedy know a way how to fix this?