Acrobat XI asks me to provide serial number every time I open it

I've been using Acrobat Pro XI for years now. We recently upgraded our other Adobe products (i.e. Photoshop, Illustrator, InDesign etc) from CS6 to Creative Cloud. I didn't upgrade Acrobat Pro, but did install Acrobat DC, though I found the tool options limited. Now whenever I open my old Acrobat Pro XI, Adobe Application Manager pops up and asks me to provide a Serial Number. I type it in and Acrobat works fine. I close the program and re-open it, Application Manager will again ask me to provide a Serial Number. I'm wondering why I now have to type in my 24 digit serial number every time I want to open a pdf in Acrobat.

Cheers.