Is your ISP OVH by any chance? Or perhaps the domain in question used to be hosted on a OVH shared hosting plan, and the migration to you plesk server is not complete yet? In that case, you would likely have to complete the migration first before being able to solve the challenge.
The key thumbprint in your response (4E3VCTFsySjUrqnCg0ooULx-3kbdPBygi0aWkvg5Gd8) matches one used by OVH for their Let's Encrypt integration. The way they have implemented the challenge verification is a bit unfortunate because it prevents domain owners from using http-01 verification. You can find a slightly longer explanation in a similar thread here:
I don't know how things would work with a OVH-provided Plesk server. Perhaps their support would be able to assist.