What is the "Share favorites across your devices" option for?
To share favorites, Maps Pro uses the "roaming" mechanism of Windows, which synchronize your applications data in the background. Your favorites are stored on your hard drive in:
C:\Users\YOUR_USERNAME\AppData\Local\Packages\29937NicolletFabien.MapsPro_27eb1sea2sgwj\RoamingState\maps-pro-favorites.json
While this is working great in most cases, but roaming is limited by Windows. The maximum quota is 100Ko. If you have a few hundreds of favorites, you might exceed this limit. To be able to add more favorites, you need to disable the "Share favorites across your devices" option in Maps Pro > Menu > Help > Options. Then stop Maps Pro. Open a Windows Explorer and copy:
C:\Users\YOUR_USERNAME\AppData\Local\Packages\29937NicolletFabien.MapsPro_27eb1sea2sgwj\RoamingState\maps-pro-favorites.json
to
C:\Users\YOUR_USERNAME\AppData\Local\Packages\29937NicolletFabien.MapsPro_27eb1sea2sgwj\LocalState\maps-pro-favorites.json
Then restart Maps Pro. Your favorites are now shared locally, so you can add as many as you like
If you need further help, please email tickets@mapspro.uservoice.com