On the Adobe Reader make sure that the SandBox Protection it's Deactivated. To do so please follow these steps:
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go to Edit
and then Preferences 
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go to Edit
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- On the categorie Security (Enhanced)
in the Sandbox Protections disable Protected Mode at Startup 
- On the categorie Security (Enhanced)
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- A warning window will appear. You'll have to choose Yes (Sim)
to Change the settings - After this operation is concluded please restart the Adobe Reader
- Please try again to digitally sign with the Remote Certificate
- A warning window will appear. You'll have to choose Yes (Sim)