This means the server was trying to log you in, but wasn't able to verify your Steam Inventory within a reasonable amount of time. (default 15 seconds)
Unfortunately there is nothing you can do about it except try again later. It might be a one-off issue, or it might persist for a couple hours.
Explanation
When you join multiplayer with skins/cosmetics equipped the server checks with Steam that you own those items. Otherwise cheaters could use items they don't actually own. Cheaters using flashy mythical items may feel like a non-issue to some players, but it is problematic if cheaters are impersonating recognizable characters to scam other players.
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.
Behind the scenes
This means ISteamInventory ::DeserializeResult returned true but the SteamInventoryResultReady_t callback was not received.