I have looked into the source code and it does not look much like a distro incompatibility, but seeing all what got commented out it looks like a half-hearted removal of some of the available actions to apply on a device ("Actions" tab).
Completing the commenting-out of the actions (attached patch) makes the hp-toolbox working again.
Committed this in the upcoming hplip Ubuntu package.
I have looked into the source code and it does not look much like a distro incompatibility, but seeing all what got commented out it looks like a half-hearted removal of some of the available actions to apply on a device ("Actions" tab).
Completing the commenting-out of the actions (attached patch) makes the hp-toolbox working again.
Committed this in the upcoming hplip Ubuntu package.