Ana, how long has she been around, and how long have the others? The questions that entered my mind were: was she slighted in any way, was she overlooked while someone else was "formally" recognized?
If things of that nature are not in this equation, I don't know what to picture, other than an in-fighting situation of a purely personal nature rather than a symptom of work politics going the wrong way for her--particularly if she "used to be pleasant." Your description of her change on a social level sounds like someone who needed to opt for an approach that is more mechanical ("I'm just going to do my job and be out of there when I'm done!") than the socially engaging person she was earlier.
My instinct (operative word, this is not science and I am not there) tells me that there is something that took place that has miffed her, and she sees it as a mature or appropriate thing not to expose that issue, while the common feeling seems to be that no one knows what they did wrong (you or your your team member individuals).
In my opinion, the approach to take depends on a few things. You say there is no issue with customers feeling bruises from her. If you are satisfied that is ultimately true, then OK, that is checked off the list. If there is anything she is doing ("doing") to create negativity toward others, then that is worth looking at, but if she is not "doing" anything besides retreating from social interaction and focussing on the mistakes of others, then she is not doing anything wrong in my opinion, she is just different and she just doesn't match the colors of every wing in the flock, and this need not be her problem, we need to respect that.
But if she is not willing to be engaged by others when it comes to collaborating on client work, then this is where she is a problem, and she would need to be questioned on what is preventing her from exercising regular interactions for the sake of a customer deliverable. (Notice how I have not used "team" words in that phrase, in the event that there is some kind of aversion on her part for the use of such words).
Eventually it WOULD come down to teamwork in subsequent conversations, as it is unavoidable that you need people to work together in order to deliver. If she is so averse to that, then she would need to be gone, as this is not going to get things done any better from the broadest point of view of the company, whether she has certain clients that appreciate her or not. She's not a one-employee company, she is in a team environment that has people that depend on each other through the fun and through the yucky.
But in HER defense: I think it is worth exploring whether there is something that you (or someone in your organization) could have missed with respect to recognition. By pointing out errors all the time, after having been a reportedly "pleasant" person, this has a ring to it for me of someone who is "getting back" for some reason.
As for the harping on errors and her diligent efforts to bring those to light: this is something that puts a damper on things, it's rotten when errors happen, and it's difficult not to feel a reaction that condemns a person for exposing these. Taking this to the brass tacks, however, what she is providing is EXCELLENCE. This is the dilemma, we are questioning someone on her interpersonal dynamics, all while she is presenting opportunities for improved success.
Typically, when someone is good at pointing out things that are wrong, that person is handed the task of figuring out how to fix it (I've been there!). I think there are limitations to that knee-jerk reaction, especially when the person has interactive challenges. If there is a hope and an expected outcome that this particular team member would stay and that this would be beneficial on all counts, then it could be that she needs to have her responsibilities modified, and maybe she becomes a QA person.
Ultimately she would need to feel happy and the team she is a part of needs that for her too, even if they are miffed at her for a "change in personality." In my conversations with her, I would want her to reach an outloud decision with me on what would make her happy, and what came out of the conversation would need to be a change in her eyes as well as the eyes watching her. Brass tacks would be that she does not have to like people in her team technically, but as an employee that will give what is needed, she is going to have to find inside herself an ability not to be grossed out at dealing with other people that need her to collaborate. If she isn't able to do that, then it's a different plan, and she will want to find a place that WILL have people she can engage. | 
