Platform: PC
ISP: Embarq
Type of Internet Connection: DSL
Internet Connection Speed:
Date & Time: previous to but last on 5/3/2011
Frequency: Every time I zone
Character Name: Eldaena Vonxandria
Race: Keeper of the Moon Miqo'te
World: Bodhum
Class/Skill Rank: Mostly on rank 50 Thaumaturge, but also on other classes before.
Physical Level: 50
Area and Coordinates: Within all zones this has happened to me. Thanalan, The Black Shroud, Coerthas, and La Noscea (I have not been to Mor Dhona recently.)
Party or Solo: Both
Monster Name: Mostly star marmots, coblyns, does, etc. Anything just outside a zoning area because that is when and where it occurs.
In-Game Time: Any
Steps:
1. Zone from any area into another one
2. Engage a mob
3. Defeat mob
4. Message saying it cannot be claimed due to the party leader being in a different area even if you are the only person in the party or if the leader is there with you.

One other thing is that I have noticed in the past you would get this message even if you were engaging in a battle with the party leader and another member was in a different zone. Not sure if this is still the case.

I hope this helps.