Same problem, for MP mission always in my client .rpt log :

Observer C Alpha 3-2:2 (PLAYER_NAME) REMOTE (SLOT) in cargo of C Alpha 3-2:2 (PLAYER NAME) REMOTE (SLOT); message was repeated in last 60 sec: XXXXX

In vehicule, but when i try to get same error i have no problem for my player.
This message is usually repeated 3-4 times and only for some players.

I have see this error when a player was inside Heli and another time inside a Strider but maybe the same problem on another vehicle.

On server with 60 players, in one hour i have aboutly 10 line "Observer C..." for 4 or 5 players.

Debian Server with Arma3 last update + AltisLife + NO MOD
No script error server/client side