What does this mean?

[lucets]

Does this mean someone keeps getting disconnected?

[MentaL]

[Purpleyouko]

No That's not necessarily what it means.

That warning message is printed when the server attempts to remove a character from a specific map.
It only happens when the requested character is not found in the current map roster.

It seems to be quite a common message in most servers and is probably due to a minor bug somewhere else in the server.
This bug results in erroneously calling function bool CMap::RemovePlayer( CPlayer* player, bool clearobject ) which you can find in MapFunctions.cpp
The message should print out the name of the character but as you can see in your own screen shot there is no name so an empty string is being passed to the function from somewhere.