Unable to complete quest (without item reward) when inventory is full

Time: 19.49 GMT + 1
Frequency: Always (as long as inventory full)
World name: Shiva
Character name: Mahe Astralis
NPC name: Vorsaile Heuloix
Monster name: -
Class/Level: PLD 47
Party or solo: solo
In-game time: -
Area and coordinates: New Gridania, Y:9, X:11
Steps:
1. Have full inventory
2. Try to complete quest "Shadows Uncast (Twin Adder)" at NPC after having done the objectives
3. Message "Quest complete" will appear but no reward will be given and NPC will still have the quest-completed-marker above his head
4. You can 'complete' the quest infinite times but no reward (EXP etc.) will be given, just the completion message will appear and the corresponding sound will play

I assume the quest checks if your inventory is full despite not rewarding any item except for seals which causes problems.