Just noted that when I'm clicking on "Open PR" in my fork, it (= listing suitable branches) works. So the other way round should be disabled?!

It worked for me here from the protobuf to protobuf (I do not have a fork).

How long did you wait? The task that's run to check which branches are ahead and thus can be turned into a PR is ran async which depending on the load on the server can takes few seconds or a little more.

I waited > 8 minutes :-).

It is a bug but it's going to be a hard one to investigate, especially since I can't reproduce it, neither on src.fp.o nor locally :(

Let me know if I can be of any help. I just looked at Firefox's network analysis tab, and AFAICT there are no network connections initiated when I click on "Open PR" at https://src.fedoraproject.org/rpms/protobuf/pull-requests, i. e. it seems to be neither a timeout nor a server error. (When I click on "Open PR" at https://src.fedoraproject.org/fork/scfc/rpms/protobuf, I see five requests.)

The console log at (working) https://src.fedoraproject.org/fork/scfc/rpms/protobuf shows the JSON representation of a protobuf object, the console log at (failing) https://src.fedoraproject.org/rpms/protobuf/pull-requests says:

Source-Map-Fehler: request failed with status 404
Ressourcen-Adresse: https://src.fedoraproject.org/static/vendor/jquery/jquery.min.js?version=5.3.1
Source-Map-Adresse: jquery.min.map[Weitere Informationen]

Source-Map-Fehler: request failed with status 404
Ressourcen-Adresse: https://src.fedoraproject.org/static/vendor/bootstrap/bootstrap.bundle.min.js?version=5.3.1
Source-Map-Adresse: bootstrap.bundle.min.js.map[Weitere Informationen]

Looking at the source codes of https://src.fedoraproject.org/rpms/protobuf/pull-requests vs. https://src.fedoraproject.org/fork/scfc/rpms/protobuf, both contain the layout for the button, but the JavaScript ($("#pr-button").one("click", […]) is only present in the latter (working), not the former (failing).

In pagure/templates/repo_master.html, including the JavaScript seems to depend on the value of g.repo_committer (line 310ff., {% if g.repo_committer %}). So … the button should not be displayed unless g.repo_committer as well? (I generally don't know what the intended UX is.)

I think I've an idea about the issue, thanks for pointing me to the right direction :)

Commit 10d747b fixes this issue