Could it be that if the challenge reads the file with the token it expects plain text and therefore it cannot proceed?
I can load a text-file without problems from directory /.well-known/acme-challenge/textfile.txt
when however I want to load the file with the token I get the question what to do with this binairy file although I just generated a textfile in Plesk.
I tried the manual way, with "certbot certonly --manual . . . " on my local machine. And I tried the plugin in Plesk the error is the same.
I run a combination of apache/nginx.