The API of the https://luftdaten.info/ project stopped answering requests yesterday night, we received the last reading of particulate matter measurements on 2018-07-04 at 00:53 CEST.
Others also seem to experience the same thing (Matthias Gutjahr auf Twitter: "Ist die #luftdaten API gerade down?"). Thus, https://luftdaten.hiveeyes.org/ currently will only display stale data.
Thanks to the people of luftdaten.info for providing this service to the community and good luck for fixing the issue!
P.S.: If you are interested in more details about what is working under the hood (LuftdatenPumpe and beyond), enjoy reading luftdaten.info — Kotori 0.22.7 documentation.