Vanilla Clients that do not have Forge Config API Port installed will receive an IndexOutOfBoundsException when trying to join the game (always with Index 44).
Viewing the logs, this is due to a Netty IO error:

[17:46:34] [Netty Client IO #10/ERROR]: Could not deserialize forgeconfigapiport:modid
[17:46:34] [Netty Client IO #10/ERROR]: Could not deserialize forgeconfigapiport:enum

Removing the mod from the server allows clients to connect.

This should be fixed in the latest version v3.2.0. If it is not please reopen this issue.

@Fuzss This issue still occurs in v3.2.0 (MC 1.18.2).