cannot delete backups with tmutil in mojave

Since installing Mojave (I think, it might have also happened in high sierra, but I highly doubt it), I cannot even do an ls into the Backups.backupdb directory. Usually I could delete several backups with tmutil. Now, when I try to access the directory, I get a simple "Operation not permitted". This never happened before, and I could delete it using sudo tmutil delete.

ls -ld Backups.backupdb/

drwxr-xr-x+ 7 root wheel 238 Jun 13 2017 Backups.backupdb/

So everybody has read access.

No idea what's going on and why it changed all of a sudden.