After an integrity check on UFoCP, this came up:
Data\XML\Units\UND\Units_Hero_Underworld_Bossk.xml:66: unknown game object "Proj_Hand_Blaster_Bossk, Proj_Bossk_Plasma_Grenade".
These two projectile types are valid, and so is the syntax. The ability works fine in-game, but the tool marks it as an error.
Tested on FoC, line 600.
_MOD_INTEGRITY.log