This is a cracked (aka "offline mode") server, so it stores user data only by usernames. If you change your username on a cracked server, it will not automatically contact the Mojang API to find out who you are, so the server in fact sees you as a completely new player.
You will need to use a cracked launcher
to join as your old username, and move your items yourself using chests. Or you can contact the staff while you are in the game, so they will open an offline inventory of your older username and give you the items.