I read somewhere in a forum, that some older graphic cards could interfere with Secure Boot in some way. And in deed, it was my graphics card. The card that caused the issues was a GT730. I replaced it with an RTX 5060 and then IncusOS got past the “Incus is starting…”.
But after a successfull installation I now get another error:
Error Failed to check for Secure Boot key updates: failed to read EFI variable “db”: x509 negative serial number (provider: images)
Any idea where that comes from?