[Python] How to obtain substring of big string text in Python?

I have the following format of text files, which are outputs of an API:

TASK [Do this]
OK: {
"changed":false,
"msg": "check ok"
}

TASK [Do that]
OK

TASK [Do x]
Fatal: "Error message x"

TASK [Do y]
OK

TASK [Do z]
Fatal: "Stopped because of previous error"


The amount of lines, or tasks before and after the "Fatal" error are random, and I am only interested in the "Error message x" part.

Code as of now:

url = # API URL
r = request.get(url, verify=False, allow_redirects=True, headers=headers, timeout=10)
output = r.text


I tried using a combination of output.split("Fatal", 1)[1] but it seems to return list index out of range, while also messing up the text, adding a lot of \n.

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