There is a problem with PitStop Inspector in Acrobat DC 2017. After starting Acrobat it is impossible to select an object with the PitStop Inspector tool.
This is also true for older versions of Enfocus PitStop. The reason for the problem seam to be a change in Acrobat DC 2017. Enfocus is working with Adobe to find a solution.
The recommended workaround is to open the window Adobe Acrobat DC > About Third-Party Plug-Ins > About Enfocus PitStop Pro… and close it. Afterwards objects can be selected as usual … until the next start of Acrobat DC 2017. Then you have to open the About Enfocus window again!
UPDATE: the problem has been fixed by Adobe in the November 2017 Update of Acrobat.