Having an issue where attempting to add an aimed block to the toolbar will boot players from the server, after picking the desired hotbar slot.

The server doesn't crash, and doesn't produce an error message. Not sure how else to provide information, so here's a clip:
https://clips.twitch.tv/InquisitivePluckyCourgetteFeelsBadMan

There are several mods on the server, but I tested it with the vanilla 'Light Armor Block' as well as other blocks. All produce the same error that boots you from the server. Am able to rejoin no problem a few seconds later, but the block is never placed on the hotbar.

Will provide more information as necessary. Cheers.