Looks all perfectly fine, also the content-length matches the actual “this works” string (+ line ending) of the file.
I verified on my home server and dietpi.com that HTTPS redirect is not an issue and HTTP/2 isn’t eiter.
See also docs: Challenge Types - Let's Encrypt
Our implementation of the HTTP-01 challenge follows redirects, up to 10 redirects deep. It only accepts redirects to “http:” or “https:”, and only to ports 80 or 443.
All fine in your case, a single redirect to (same) hostname to HTTPS on port 443, just standard.
Can you access that URL as well via browser from outside your LAN? E.g. try with mobile phone and mobile Internet used. It should show “this works” in browser window. You can create another file with shorter name, to avoid typing the long random string 
.