I know its a bit late for you but I recently had this problem and found this thread hoping to find a solution to no avail. I've returned to this thread to shed light on anyone else who runs into this problem in the future.
Some mods have this issue where they work fine in singleplayer but in multiplayer can do things like this. I had this problem with the "Chest Browser" mod and only discovered this by removing every mod 1 by 1 and opening the server again every time.
If you have this issue check if you have a mod called or similarly titled "Chest Browser". Removing/disabling this mod from the server and the users' clients should fix the issue. If you don't have this mod, I recommend, however tedious, going down your list of mods and disabling them either 1 by 1 or in groups until you find the problem mod(s). It shouldn't take too long and if you find another mod that causes a similar issue please let everyone know.
This was on tModloader 10.1.5