We had the exact same error when try to cloning into a new deployment slot.
Since AccessKey would normally only be used in Storage Account, we looked around and found
Settings -> Configuration -> Path mappings -> It was mounting a storage account (Currenly only avaliable in Linux App Service Plan)
Since we are not using this mount, we removed it and tried cloning again.
And it worked this time. But yes, I agree someone need to open up a ticket with MS to have this fixed permanantly.