Platform: Windows 7
ISP: Forthnet Hellas
Type of Internet Connection: aDSL
Internet Connection Speed: 24/1Mbps (Connecting with 13524/1020 Kbps)
Date & Time: -
Frequency: Always
Character Name: Ziny Psxlover
Race: Lalafell F
World: Lindblum
Class/Skill Rank: Conjurer/Thaumarge
Area and Coordinates: -
Party or Solo: -
NPC Name: -
Monster Name: -
In-Game Time: -
Steps:
1. Go to Configuration -> Keyboard and set any key for "Accept/Confirm"
2. Try to use that key on some other option with Ctrl or Shift
It says "Unable to map. That key is already being used."

Turns out the information on http://forum.square-enix.com/ffxiv/threads/33363-Setting-Page-Up were wrong. It wasn't the keys, it's that whatever key you've set for "Accept/Confirm" can not be used in anything else no matter the compination. So for example if you set Page Up for "Accept/Confirm" you can't use it anywhere else as Ctrl + Page Up or Shift + Page Up.