I'm having the same issue, verified running on version 1.20.4 with Xskills v0.8.14 and Xlib v 0.8.13. For some additional information, the mod works fine if I launch the world via the game client, but the dedicated server doesn't load either Xskills or Xlib when launched.
Edit: Downgrading both Xlib and Xskills to the previous release (x.12 for Xskills) has caused them to show up on the dedicated server.
However, when smithing an item, when the work item is finished I get booted from the server with the following error from the server
Edit Again: Poking through the logs more I found that both updated versions of the mod were put on the server's disable list for some reason. Removing them from that list has allowed them to load into the server and they seem to fix the exception issue that was being thrown.
For anyone having the same issue, the file to change is serverconfig.json at C:\Users\username\AppData\Roaming\VintagestoryData
Look for the DisabledMods value and remove Xlib and Xskills from it.
