Thank you for reply!
This information is really valuable. So, looks like the problem appears to be not in my particular client.
Well, the firefox greasemonkey timeline extension (Wanikani Ultimate Timeline) works fine. So, nothing wrong seems about server side, assuming the API used is the same.
Hmm… The mentioned app last update was 2 years ago. I doubt I can get in touch with any developer.
Hmm… Okay, looks like I should just hope it will fix itself someday.
upd: It actually may be consequence of some server update and the way this app uses (probably) obsolete 2 years old API calls. Then, it’s broken for good.
upd2: Seems like this application is an open source project so I actually can look up for the issue source. I’ll do that if it gets too annoying without this feature.