Friends can't join me and Marketplace isn't working properly!

My friends and I started a nice survival world on my PlayStation. All of my friends could join my world. Since 2 weeks, all of my friends are unable to join my world, but somehow I can still join theirs. I tried to re-install Minecraft multiple times, but without success. It is strange because when they try to join my world they would be stuck on “Locating server” or “server not found”. Somehow, I can still see their names on the player list, but I cannot see themselves on my world. I also noticed that the marketplace isn't working properly. On the main menu, there is always an error message like “Invalid session”.