ISP: Plusnet
Type of Internet Connection: DSL
Date and Time: 12:30pm GMT
Frequency: Happened 3 times when I have played Aurum Vale in the last week
Character Name: Cuvelia Stellarum
Race: Lalafell Female
World: Shiva
Class/Level: Level 50 White Mage
Area: Aurum Vale
Party: Party of 4 members
Monster name: Coincounter

Steps
1) Join an Aurum Vale party (Healer I feel works best to reproduce this bug)
2) Try to heal people and sometimes the heal will not register on other players despite everything else including damage and debuffs registering fine.