Resolving the problem
Disable the Adobe Reader setting "Enable Protected Mode at startup".
Steps:
Ask the users to perform the following steps:
Launch Adobe Reader
In "Adobe Preferences" open menu "General"
Untick (uncheck) the setting "Enable Protected Mode at startup"
Note: If you find that the option to enable/disable Protected Mode is itself grayed out, consult the below linked Adobe support article that discuses the Registry and GPO (Policy Editor) options.
Related information
Protected View in Acrobat X Version 10.1