This means the server was trying to log you in, but wasn't able to verify your membership of a Steam group within a reasonable amount of time. (default 15 seconds)
As a temporary workaround you can clear your character's Steam group:
- Open the Unturned main menu
- Click Survivors on the left
- Click Group
- Click the name of your current group next to the Group label to clear it
When your character is associated with a Steam group and you join multiplayer the server has to check with Steam that you actually are a member of the group. Otherwise cheaters could claim they are a member of your group to break into your base.
Usually it happens when the game server has temporarily lost connection to Steam. Especially on Tuesdays when Valve performs Steam's weekly maintenance.
There is nothing the server host can do about this unless it has been happening for a long time (say, 8+ hours) in which case there might be a problem with the server's network configuration / firewalls.