Also have this issue - in my post I've found that this is potentially a version issue, our current running version is an old snapshot, i'm hoping this is the issue.
I have the same issue! In my latest forum post - I noticed the mcmmo is out of date - an old snapshot, this is likely the issue. I'm hoping someone notices my post as well and fixes this.
Hello!
I'm seeing that the current MCMMO is showing 2.1.100 - snapshot far out of date - and shows issues with /party commands for me.
Thank you so much!
Guccius