I wanted to move my nixOS installation to a bigger SSD. After copying over the data, and creating a UEFI boot entry, it didn't show up in the boot menu. After rebooting I found the entry changed to:
This seems to be the indication for a broken entry. In my case, the contents of the EFI partition weren't right, but this also comes up, after I disconnected the original drive. So it seems to be the sentinel value for an entry for which the UEFI can't find the target.