Right now, only Trustees have the permission to place lava buckets.
Would be cool if someone with the neccessary powers could fix that.

I have looked into the issue as far as i could, and found out the following:
The permission which allows Trustees to use lava buckets is "essentials.protect.exemptusage" (in interplay with " essentials.protect.alerts.notrigger" to not trigger warnings to all Trustees).
So as far as i can tell, the usage of the lava bucket (item) is forbidden in some essentials blacklist - which i cannot edit from ingame.