I see this problem from time to time.
Cannot download this url for example; https://f-droid.org/repo/org.droidupnp_15.apk
Is this some server/mirror failing?
$ curl -I "https://f-droid.org/repo/org.droidupnp_15.apk"
HTTP/1.1 404 Not Found
Date: Tue, 12 Sep 2017 17:26:05 GMT
Server: Apache
Strict-Transport-Security: max-age=31536000; includeSubDomains
Content-Type: text/html; charset=iso-8859-1
Via: 1.1 f-droid.org (Apache/2.4.10)