My Adobe Acrobat Standard DC does not launch

My Adobe Acrobat Standard DC (2015 release, standalone, activated in October 2015) had been working properly.  A couple of days ago, when I tried to open a .pdf file, a dialogue box popped up requiring me to sign in.  After I signed in, another dialogue box popped up requiring me to subscribe.  It did not allow me to open any files.

I uninstalled the software and then reinstalled it with my original serial number.  The re-installation was successful.  But, the software did not launch.  When I clicked the icon, no response at all.

As I tried to run the software as administer from Acrobat Distiller DC, an error message popped up saying, "Your request could not be completed.  Adobe Acrobat license has either expired or not been activated."

As far as I understand, the license that I purchased is not a subscription version, and obviously it was already activated.  I would like to know what causes this problem and how to resolve it.  Thanks for your help!

Additional information:  The above-mentioned dialogue box that required me to subscribe was to subscribed to Adobe Acrobat Pro DC, not the one that I have (Standard DC).

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