If the vanilla player drops any item in (fabric) L4J server, they instantly disconnect with the message on their screen:

Internal Exception: io.netty.handler.codec.DecoderException: java.lang.IllegalArgumentException: Can't find element with id 1542

Vanilla players near the dropped item (or when the fabric player drops it near them) get disconnected too, but anyone who uses the mod are unaffected in the server.

Tested this on my test server (with only architectury, l4j, and fabric api installed) running in my computer, this is what it actually happens on different clients (one for fabric, and one for vanilla (no optifine or mod loader)).

On server console, my alt player simply return "lost connection: Disconnected" during this, and there's no any error messages happening on it.

Fixed in version 1.6.3, thanks for reporting.