Uptime Robot has fixed the error. The monitoring works again as usual.

The UptimeRobot API is currently down which means the app is not able to display real-time information. We have been in touch multiple times with their support and they keep telling us that it should be working "very soon". Unfortunately, their own status page at status.uptimerobot.com does not represent the actual state of the API.
The good news: Monitoring works and you will receive e-mails, just the visuals in the app are currently not working.