Try and fork a namespaced project that you don't own, and you get a 404.
I forked modularity/container-image-template , Pagure tried to load https://pagure.io/fork/adamwill/modularity/container-image-template , which looks logically correct, but gives a 404.
(When you fork a namespaced project in github the namespace is dumped, but I don't know if you want to go with that approach).
Looks like @pingou is aware of this, he mentioned what sounds like the same thing in https://pagure.io/pagure/pull-request/1522#comment-12961 .
Commit 3e8be541 fixes this issue
@pingou changed the status to Closed
Closed