That URL seems to be on the hosts-file.net adlist.

/etc/pihole/list.7.hosts-file.net.domains:30197:127.0.0.1       hockeyapp.net
/etc/pihole/list.7.hosts-file.net.domains:36230:127.0.0.1       rink.hockeyapp.net
/etc/pihole/list.7.hosts-file.net.domains:36779:127.0.0.1       sdk.hockeyapp.net

Try manually whitelisting the URL using pihole -w

@manuelgu Thanks so much. Call me stupid for not looking into that but I never thought that it could be blacklisted since it serves beta apps and no ugly ads or trackers.

hosts-file sometimes adds legitimate domains to their lists...

If you take a look at their listing for the domain, it's marked as ATS which is defined as:

TS - Ad/tracking servers
This classification is assigned for domains being used for advert or tracking purposes

Could be a false positive, or it could be that they're hosting tracking ads! Does it host apps?

@PromoFaux It hosts apps for beta testing for selected testers. The only tracking I might think of is for improving user experience and finding bugs. And the beta testers would be aware of that when they sign up for beta testing.
I see that "sdk.testflightapp.com" (testflight is another similar beta-testing distribution platform) is also marked as ATS on hosts-file.

I'll try sending the hosts-file person an email and ask for removal of hockeyapp.

It may even be that an app hosted on their servers was malicous...

@PromoFaux That's a possibility. It's not a reason to ban the whole thing though because they apps are mostly handed out to select beta testers. Anyway, I emailed the hosts-file.net guy and he removed hockeyapp from his list. Thanks a lot.

Super! Did they give any indication as to why it was blackisted?

None. He just said: "Thank you for your e-mail." and "The entries have been removed."
¯\​_(ツ)_/¯

oh! Almost worrying that it is that easy....

I guess he must have checked. HockeyApp has always been trustworthy, and now has been acquired by Microsoft.