Ah yes, Marara
She is part of a vampire quest, you can only do her quest if you are a vampire yourself.
If you are not a vampire she is hostile and you have to kill her.
So then you then cant do that quest when youre a vampire.
But thats not a case of a broken quest, thats an alternative way of completing the quest.
You still get the exact same reward, the very nifty Marara's ring, and the end result is the same with Marara being dead.
All you miss out on is a log entry.
Thats how Morrowind is, there are a lot of ways to get the same result and one isnt neccesarily better than another.
I had found Marara games before by chance I once went to her lair as a vampire.
I found her not being hostile and hearing her story a fantastic little bit of 'extra content.'
I was over the moon and went out to find all the other 'lone hostile vampires' in the game. Its how I discovered the Spellbreaker.
Morrowind rewards you for trying out things in different ways, seeing what is possible and I love that.
A broken quest would be that Dunmer lady in Balmora who wants rats cleared from her attic.
She can give you an 'extra comfy pillow' if you find an invoice of her in a shipwreck, and bring it to her so she knows what happened to her shipment.
Due to a bug alas the option to talk to her about an invoice dissapears when her rat quest is completed.
Its likely one of the first quests players will do and the invoice is in a rather obscure location so many people never even know there is such a quest.