Ever since I upgraded from a demo account, whenever I log on to any multiplayer server, I get error messages such as those below:

SKIN : No file size info - XFG_LP4_Advan
SKIN : No file size info - XRG_Giger
SKIN : No file size info - XRG_Barricade

No one's skins are visible to me anymore; just white cars everywhere. Help?

Try emptying out your data/skins_y folder.

It sounds like there's empty files in that folder, that LFS can't open

I emptied the data/skins_x folder and the skins_y folder was already empty. Unfortunately, the problem persists. Thanks though, was worth a shot.

Is your skins_y folder by chance read-only?

No, it's not read-only.