CS6 Perpetual license: Expired trial issue

I have a CS6 perpetual license.  My laptop's motherboard had to be replaced a couple of months ago.  After the replacement, every time I try to open Acrobat Pro X, I am prompted for my login and serial number.  When I enter these, I can usually open the document, but if I do not re-authenticate every 20 seconds or so, the program shuts down.  No updates or changes have been made to Acrobat for at least the last year.  Advice on how to get this working again?

I have browsed the forums for an answer, and many seem to focus on Creative Cloud issues.  The closest one to the problem I am experiencing is this:

https://community.adobe.com/t5/acrobat-discussions/adobe-acrobat-pro-2017-have-perpetual-license-but-says-trial-period-has-expired/m-p/9853753

The user mentions that an Adobe tech changed the settings for him.  But no further information...

Your help is much appreciated.