I don't see the relation to this bug tracker.

Probably we need to do something?!

Who is "we"? I have no access, and don't want to maintain this actually. IMO it's the duty of the OBS folks to fix this stuff.

"We" is really anyone who is interested enough in the AppImage project and reads this here. (Read: I don't have the time to check for and keep up with OBS changes in a timely enough manner.)

Then please open an issue wherever that is. Here it'll just get lost...

If OBS makes changes that require the AppImage project to do something, where should I open an issue?

What does "require the AppImage project to do something" mean? Why don't you open an issue in the OBS bugtracker or send them a mail? They've implemented everything. You should talk to them first, and then open useful issues here if there's really stuff to do. This issue doesn't contain any "we need to fix" things. It should be closed as invalid

Well at this point I don't even know whether the breakage is maybe just temporary. It is a placeholder that someone needs to investigate what is going on.

Still has nothing to do with the development of AppImageKit...

What it's basically saying is that there is no tag called appimagetool/master in the git repo. Was there ever one, or should this be just master?

Was changed several weeks ago to master.

Then whoever is responsible for the package on OBS will need to update the _service file to point to master instead. The error message is quite frankly terrible but it's not a bug in OBS itself.

Then whoever is responsible for the package on OBS will need to update the _service file to point to master instead. The error message is quite frankly terrible but it's not a bug in OBS itself.

That would be the OBS folks. You need to reach out to them, e.g., in #opensuse-buildservice on Freenode.