sure - It took me some time to create a reproducible testcase. In the end, it is the missing incus-agent that provokes the error.
btw - since the incus-migrate seems not to configure/install the incus-agent, what are the “official” steps to install it? I found Ship multiple VM agent binaries #263 but not how to do the bind-mount. For my tests I downloaded the deb package from debian sid and installed it inside the ubuntu vm. When creating a vm via cloud image I see incus-agent running from /run/incus_agent/incus-agent but I don’t yet get how this is beeing mounted/started.
→ I created a new question for this: How to install incus-agent on migrated systems?