Xsane and hp-scan work fine here with a HP Photosmart 6510 (no plugin needed).
Can you try upgrading to the latest version of hplip-3.12.10a (do not forget to upgrade the binray plugin as well). Then restart cups, delete and re-configure all printer queues. Please report back if this fixes the problem.
You can also try if a revdep-rebuild fixes the issue.
Xsane and hp-scan work fine here with a HP Photosmart 6510 (no plugin needed).
Can you try upgrading to the latest version of hplip-3.12.10a (do not forget to upgrade the binray plugin as well). Then restart cups, delete and re-configure all printer queues. Please report back if this fixes the problem.
You can also try if a revdep-rebuild fixes the issue.