Cannot check, save or delete Cron Events

In the past, whenever making staging copies of https://diespitze-coaching.de for development and then returning them to the live environment, I went and checked existing cron jobs and they were marked “Needs checking”:

https://www.pasteboard.co/BlMVqQxBS6rc.png

As you can see at the bottom of the screenshot, I made a new cron event today because when I click “Check and edit” on any of the top 5 and then click the “Update event” button, they’re still labeled as “Needs checking” on the overview screen just like you see on the screenshot. When I delete them, the screen refreshes and they’re still there.

I seem to recall that this was the same the last 5 times, which is why there are now 6 identical cron events. I thought I had reported this issue before, but couldn’t find it. So here goes nothing, I’m reporting it again.

Any way to at least get rid of these old cron events?

The page I need help with: [log in to see the link]