Acrobat X Standard - serial number is not working

I have used adobe acrobat for years.. maybe average once very month or 2 to just join files, maybe make a form fillable and fill it... When i installed it, it took the serial number and worked.. then every time i use it after that it no longer works and says "this serial number is not valid for this product" i am using the same install file i used years ago (i downloaded and saved it as well as my serial number) - the install is English (north america - now labeled as trial).  I DO NOT WANT to go to a subscription.. it would be a terribly expensive solution for my use scenario.. Just a month ago, my old laptop worked just fine but when i upgraded it won't anymore.. 

In addition i will note that i do not have a secure program as it hasn't had updates.. So i do not use it as default and only open a few specific pdf files with it.

What is the solution?