My buddy and I have the identical problem. We both play on the PC and can't invite each other. While trying it out, we came across the "supposed" problem. If you are in the main menu of Rainbow Six and you open Ubisoft Connect, click on your profile in the upper left corner, you will see the following:
Offline - then you can click on "Set Online Status". There you should actually choose between "Online", "Do not disturb", "Absent", "Invisible". However, the fields are grayed out and you can't change anything there. Unfortunately I can't upload a photo of it because I get an error..