"set display name" doesn´t work on a server

Hello everybody :) I need your help.

In my Modpack there is an item which opens a GUI on rightclick. There is a button, a textfield and one slot in this GUI.

When you press the button, an item appears in the slot with the content of the textfield as its name.

If this is tested in singleplayer, it works fine. However, if I export the modpack and try it on the server, the item appears in the slot, but if hover over it the name it didn´t get changed and if  I want to take it out of the slot, it just disappears.

That happens to every item i want to rename (set display name in a gui).

What can i do about that? The code works as i said but not for the server :/

Thanks for your help! :)