Maybe this will help some:
On the 17th of April I played for a few hours with my son in co-op. I was playing on PS5, he was on PS4. We both acquired some trophies. We had no issue at all.
Today we tried, but he just couldn't connect to a Ubisoft server. I had no trouble on PS5 but my son kept getting an error code message ("sorry the required ubisoft services are currently unreachable", not sure of the exact error code). Either I invited him or he just tried to go online, it wouldn't connect.
So I closed the game on the PS4 (it's in the next room), tested the internet connection, reloaded the game, same result. I closed the game again, then decided to restart the PS4 but accidentally sent it to sleep. After waking up and logging into my son's account, it started the rare but usual profile update thing when the Sony network EULA or something changes: who can see your real name, who can send you invites (friends, friends of friends) etc. After we confirmed our way through these settings, I finally restarted the PS4 (this step might have been unnecessary), loaded AC Unity, and voila, everything worked just fine; we could play in co-op again without problem. Hope this helps.

Maybe this will help some:
On the 17th of April I played for a few hours with my son in co-op. I was playing on PS5, he was on PS4. We both acquired some trophies. We had no issue at all.
Today we tried, but he just couldn't connect to a Ubisoft server. I had no trouble on PS5 but my son kept getting an error code message ("sorry the required ubisoft services are currently unreachable", not sure of the exact error code). Either I invited him or he just tried to go online, it wouldn't connect.
So I closed the game on the PS4 (it's in the next room), tested the internet connection, reloaded the game, same result. I closed the game again, then decided to restart the PS4 but accidentally sent it to sleep. After waking up and logging into my son's account, it started the rare but usual profile update thing when the Sony network EULA or something changes: who can see your real name, who can send you invites (friends, friends of friends) etc. After we confirmed our way through these settings, I finally restarted the PS4 (this step might have been unnecessary), loaded AC Unity, and voila, everything worked just fine; we could play in co-op again without problem. Hope this helps.