Because single player is only your game, on multiplayer there are other people
For example, I remember downloading this stretched out truck for tbogt (in which you could stand in the back, which you normally couldn't). For single player it's no problem but in multiplayer when I stand inside the back it's conflicting because 'my game' says that's possible while the files of the other people say I'm doing something that's not possible (because I shouldn't be able to stand inside the back of the truck). Even spawning the truck was enough to get the error you mentioned.
Skins (weapons, player skins etc) are never a problem but when you change something structural that differs to much from the default situation, you will get that error every time you play multiplayer.
Do you have EFLC? If so I'd recommend using the basic GTA 4 for modding, and keeping Tbogt/Tlad clean for multiplayer (other then perhaps weapon mods and weapon sounds for example). Or the other way around ofcourse.