How many hours is your save game at when that happened? Just a thought I had, my companions seem to be having responsiveness issues showing up now after 20 hours. Then again, it was also in fights against gobbos in their main fort for me and sounds like maybe similar to where yours happened. So it could be that area has an issue too.
It wasn't Astarion for me, it was my main PC. Stopped being able to confirm abilities to activate them (weird delays and then nothing happens). And then I couldn't end turn a few fights later (I forget which character that was). Reloads fix both for me too.
