I am using printer mod version 3.2.1-sakura.2 for 1.21.4 and I often gets kicked from the server when printing. Here is the server console output:
[09:29:29 WARN]: ImLunchtime tried to set an invalid carried item [09:29:29 INFO]: ImLunchtime lost connection: Invalid hotbar selection (Hacking?) [09:29:29 INFO]: [voicechat] Disconnecting client ImLunchtime
The server runs on Paper 1.21.4 and I have OP and are in creative mode. BTW I don't have any Anti-Cheat plugins installed.

Hey, this plugin seems to work to stop that: https://www.spigotmc.org/resources/invalidhotbarfixer.122180/