The way ChatItem used to work was by sending the client the data of the item as a string. To achieve that, the plugin used reflection to get the item's NBT Tag and call the toString() method on it. However, that method did not escape special characters or ignore invalid NBT Tags. Therefore, tag keys containing values with special JSON meaning, like '['s, ']'s or '{'s, '}'s, ','s and ':'s, would fail to be properly parsed by the client, therefore the client resorting to drawing the "Invalid Item!" string instead of the proper item representation.
In this new version, I have created a new method to replace the NBTTag#toString() that ignores invalid tags and other tags specified in the list of ignored tags, and which properly escapes any special characters.