Possible bug? "no bootable paths with current EFI signature database"

Trying Ubuntu Core 20 (image created by myself) with secboot and disk encryption enabled, I get following fatal error message twice on initial setup:

```
taskrunner.go: 271: [change 2 "Setup system for run mode" task] failed: cannot make system runnable: cannot seal the encryption keys: cannot add EFI secure boot policy profile: cannot compute secure boot policy profile: no bootable paths with current EFI signature database.
```

The message is the same as in https://bugs.launchpad.net/intel/+bug/1938678/comments/39 .

In my setup, the UC20 disk is not the main hard drive (but making it the only drive does also not help) on the amd64/x86 box. I added the UC20 boot entries manually to the UEFI Boot setup.

Any idea if this is a bug in secboot or if some other system settings etc. need to be changed?