[2021] Unable to invite friends

Hi there,

i've tried to invite a friend to a multiplayer match (coop) but everytime only get the message that my invite couldn't be sent.
We've then tried to set the match to "public" and she could join via the overlay.
But today we wanted to continue to play and the loaded savegame was set back to "non-public" and the button is greyed out.

In other words we are both not able to continue our game session!

Things we've already checked and tried:

- Both tried to host a multiplayer savegame and invite the other person
- Reinstalled the game and the launcher after clean/backup everything
- Checked our TLS 1.2 availability which results in that our systems have it installed AND activated
- Searched for a Savegame Editor to set the savefile back to "public" but couldn't find a suitable program
- Updated our System in case we are missing any Updates or Packs of .NET, Drivers, etc.
- Searched for hours in the web to find a solution but sadly found nothing helpful

And now i've ended up here to ask for a solution because the problem seems not to be on my or my friends side.