Troubleshooting

UTF-8 codec can't encode characters ... surrogates not allowed

Dropbear host keys are binary files and not standard OpenSSH keypairs. Older Python/Ansible versions cannot handle these key correctly.

ERROR! 'utf-8' codec can't encode characters in position 23-24: surrogates not allowed

Upgrade to Ansible 2.20 and a recent Python version. Ansible 2.19 is considered a breaking change therefore if you are upgrading one, upgrade the other.

I must use Ansible <2.18 or older Python versions

Dropbear keys cannot be stored encrypted in {host,group}_vars directory. Ansible will try to automatically decrypt this file and fail due to UTF-8 encoding issues on older versions.

Storing within ansible but outside of {host,group}_vars to prevent decryption until the binary file is copied, wherein the decryption happens correctly.