Photos system library keeps getting reset

I'm using MacOS High Sierra. Whenever I launch Photos, it will bring me to the Welcome Screen, which is because I haven't set any system photo library. I can't. Each time I go into Preferences and set my photo library as the system library, it gets reset when I quit and relaunch Photos. I've tried rebooting the Mac as well. I've also tried to remove the com.apple.photo folder and the Preferences as well. No luck there too.

I've tried creating a new photo library, and setting it as the system library. When I launch Photos with this new library, all is well and good: no welcome screen and the system library stays there.

So, no problem setting the system library with a new photo library, but can't do it with my old library. I've been using this same library throughout all the upgrades from Leopard to Lion to El Capitan to High Sierra. Perhaps along the way it had some issues which I didn't detect. I have no problems using it to sync photos to my iPhone, etc. I don't use iCloud library (my photo library is 88GB).

I would hate to have to export all my photos from my old library and import them into a new library. I would have to redo all the face recognition, and I'm not sure if the geo-location will be kept. Furthermore it will be challenging to export 88GB data on a hard disk with limited capacity.

Is there something about the photo library that I could fix?