StreamSerializer doesn't work anymore

[mauricelps]

• This issue is not solved in a development build

Describe the bug
I utilize ProtocolLib's StreamSerializer Class to sync survival server inventorys across servers.
Up to 1.20.6 everything worked like a charm, however upgrading to 1.21 it does not work anymore.
I then tried to take the plugin, which I was using before (which also used ProtocolLib) and it does throw the same issue.

To Reproduce
Steps to reproduce the behavior:

1. Join the server
   2.) Put Items in the Enderchest / Inventory (Inventory or Armor Slots)
   3.) Change server to any other synced
   3.1) Error on the server where the player was leaving from
   3.2) No Error on the server where the player is joining on

Expected behavior
See above, starting at Step 3 - there should no error be on com.comphenix.protocol.utility.StreamSerializer

Screenshots
Stack Trace of said error: https://pastebin.com/QL55BdYG

Version Info
Provide your ProtocolLib install info with /protocol dump through pastebin.

Additional context
Add any other context about the problem here.

[JustRed23]

Fixed in #3217