I can no longer hand off images from Lightroom 6 to Photoshop CS6. It comes up with an error stating “the file could not be edited because Adobe Photoshop CS 6 could not be launched. It actually does launch but the image won’t come over till I click OK on the LR error and then it launches.
I assigned CS5 as the “additional external editor” and that works fine with a handoff in LR.
Try resetting the preferences of the Photoshop.
Note: Using the keyboard shortcut, preference files for custom shortcuts, workspaces, and color settings will also be reset to default.