Same issue as reported here

ISP: 1&1
Type of Internet Connection: ADSL
Internet Connection Speed: 3500kbps
Date & Time: since 1.20b
Frequency: random
Character Name: Crim Soukyuu
Race: Lalafell Dunesfolk
World: Trabia
Class/Level: CNJ50
Area and Coordinates: Bowl of Embers (Hard), Thornmarch, Nophica's Wells
Party or Solo: Party (leader at one party, member at the other 2)

What happens is, you are suddenly not able to navigate the members list in the up direction and navigating down skips some party members as if they are dead.

Example:

P2 - targetable
P3 - not targetable
P4 - not targetable
P5 - targetable
P6 - not targetable
P7 - targetable
P8 - not targetable

All party members are in range, clicking on the member in the list with the mouse works. Target type 1, filter set to "enemy" (left/right - enemy, up/down party members)

Re-logging fixed it for me, but it came back later. Started happening since 1.20b was released.