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?