Seems that the Bug Correlations view broke at some point. Given that no one is noticed, I think maybe it should be removed for now until it is improved to a point where it's actually useful.
I noticed I couldn't get it to work the couple of times I tried over the last month or two; but I don't think I ever used it when it was working, so wasn't sure if this was expected or not. Sorry for not filing a bug at the time!
Wontfix in favour of OrangeFactor v2 which will consume Treeherder's API, and likely be written from scratch.