Renate Im so sorry. I just read this now and I had no idea. I can feel your pain and it took me back to the night I lost my preious grey Koki. Almost the same details, she was great, suddenly stopped eating, diarhea, then vomit and even after a visit to the vet (a stupid one) she died in my arms and it was very painful. I got another one and I did blood tests and the vet at the hospital confirmed PBFD. I sent her back to the shop cos this is not treatable and it will kill eventually.
What you have to do now in case you wanna get another bird is a very thorogh cleaning and disinfecting of your home as this disease can linger in your home for months and affect another bird.
Please let me know if you need help in that, I will give you all details
I am so sorry again and wanted you to know that there is nothing you couldve done diffeently, its just Gods will.