Forgive me for limited knowledge, could it be apart of the pal/utc region coding? Same as you can't buy a game from another country and play it in your home country? (Eg. buy from USA and play in Australia)
Dont know why, but its very strange. I, as a german, cant play on my austrian friends account via shareplay, but he can play on mine.
Pretty annoying because he struggled a lot with maelstrom and I couldnt help him, not even watch to give tips.