I can't join a friend who created a server via essential mod. I think it's because of the character creation screen, because the game can't load it for the joining player. Could it be fixed?

Not related to this mod.
That is a bug in another mod called U-Team-Core.
This PR fixes that: MC-U-Team/U-Team-Core#256

The developer released updated versions for 1.19.4 if you're on that version.
Here's the downloads for 1.19.4:
Forge - https://www.curseforge.com/minecraft/mc-mods/u-team-core/files/4509830
Fabric - https://www.curseforge.com/minecraft/mc-mods/u-team-core/files/4509829