I have had this problem in the past…ive unfortunately forgotten what the cause was.
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i would check to ensure your webserver permissions to the directory in question are correct
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Has the directory outlined in the error above actually been created? If not then that may help in the problem solving process.
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This may be a result of my inexperience, but can i throw it out there anyway…i note the URL in the error is http …shouldnt it be putting the challenge in https?
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Have you opened port 443 for https? (once again i dont know what error this throws in terms of the acme challenge passing)
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Are you running nginx? Is it configured to use the same directory as lets encrypt?
https://community.letsencrypt.org/t/404-error-at-well-known-acme-challenge/44271/6 -
These posts may be of help as it appears to throw same error as yours…
https://community.letsencrypt.org/t/unauthorized-404-error-well-known-acme-challenge-on-a-hosting-web-space/55512/4
https://community.letsencrypt.org/t/404-for-well-known-acme-challenge/38381