This mod seems to have issues working with Project MMO or PMMO.
I haven't entirely dug through the crash log to see where/what the issue is, but the biggest indicator I have is that PMMO also does attribute stuff and upon trying to login to any save you're just kicked back with "invalid player data"

The custom modpack I'm making works with PMMO by itself, or your mod bundle (skills, skill trees, and attributes) itself, but not both so that's where I'm able to (I think) single out some sort of incompatibility and not using a mismatch of your mods either since it'll load with them just fine too.