Can't delete/modify some contacts from iCloud contacts

I have some contacts in my I cloud account, that I can't delete.

If I delete them, the keep coming back. There are only visible on the iCloud website and not on any of my devices. I only have an iPad and in the contact app there are no contacts shown. But in the iCloud web interface there are these 21 contacts that I deleted several times, but they keep coming back.

When I try to modify them, it says:

"Your changes to this contact couldn't be saved because it was just updated by another device."

(see screenshot)

But I have not other device that is synced to the iCloud contacts.

So how can i remove these contacts?