I have experienced similar situation, a player could be targeted but could not be cured. ^^;

He was standing beside me on my screen, I could target him normally.
But any spells even Protect would not reach him, It should be his problem bcoz no one was able to buff him.
Apparently his position on the server was out of sync, his postion was not in the range of our spells.
The problem was fixed when the fight has restarted.