Thank you for the quick answer.
After a short break in playing, we found some time to try to figure out the issue following your suggestions.
We found a way to play coop consistently, that is tricky but successful almost every time (i don’t know if anybody will ever have the same issue, and need this, but i’ll post it just in case).
1) Player 2 (aka, the one who plays the online coop campaign as a GUEST), creates an arena lobby and invites Player 1 (aka, the one who plays the coop campaign as a HOST).
Both players must have the connectivity set to “Public”.
Often, Player 2 needs to send at least three invitations before Player 1 can join the Arena lobby (after accepting the first invitation, Player 1 is often stuck on a screen with a message that says “connecting”; he should leave the arena menu. The second invitation often results in failing to join the Arena lobby. Finally, the third invitation is often the successful one).
2) After Player 1 has joined, Player 2 leaves the lobby.
3) At this point Player 1 hosts a new campaign, and invites Player 2 to join. If player 2 can’t join, you should restart the whole process from the get-go.
4) Once the new campaign has started, Player 1 loads the campaign the two had trouble playing.
Thank you Raze for your time and a special shout-out to the whole Larian team for creating such an amazing game.