Hi, so my friend is trying to join my server with SimplySwords installed, and every time he attempts to join it, he gets this really strange error message.

https://cdn.discordapp.com/attachments/1030876429111525406/1104829979172278404/image.png

The message is in the screenshot.

These are the deprecated effects that were removed in the latest version of the mod.
This means that your friend is trying to connect with a newer version of the mod than the server has installed.

He will need to downgrade, or the server will need to update.