Finally, success! My Packer Ubuntu Image creation works. The fix was switching to SFTP instead of SCP for file transfers in Ansible. I currently think that the SSH Proxy Packer uses by default is at fault somehow.
Now researching how generic netbooting works to configure my Udoo x86 II with it. Seems it is a bit different than the Raspberry Pi netboot I've already set up.
The social network of the future: No ads, no corporate surveillance, ethical design, and decentralization! Own your data with Mastodon!