How to Solve Glitch Preventing Me From Attacking!

I'm a free to play player who's playing a Jedi Knight, 50th level. On a mission called Firestorm, on the planet Corellia and at a place called Korvalus Towers I am unable to attack the Sith Stalkers in the game but am fine to attack other opponents.

When I click (any) attack button while focused on the Sith Stalker I get the message, "Out of range. Move closer." This happens when I'm right up next to the Stalker. Eventually I die of course and start back at the medical centre - and in this way cannot advance throught the game as this is my Primary (or is it called Class?) Mission.

Anyone else heard of this problem and any ideas how to fix it? (No customer service for FreetoPlay)

All help appreciated!