We are having exactly the same issue.
Due to a power outage, my PC shut down, and my server files got corrupted, mainly the main.ttw and main.ttw.bak.
After searching through the internet, I found a solution to replace these old files with new ones.
Luckily, I had the starting world as a backup, so I copied those files, and it fixed our issue.
Now we are having a problem with the clear zombie quest; after 2-3 failed tier 6 runs, we decided to do some tier one so that we could search the whole area to see if there were any zombies hidden or something. Still, tier one was also not completed.
Then, I found a solution on Reddit that a restart might solve the issue. Luckily, it did.
But now, we have to restart the server every time to complete any clear zombie mission.
The fun part is if we do the quest alone, there is no issue whatsoever, but when we try to do the quest with the team, it won't clear.
