I can't really say since when this started happening because it's been a while since me and my friend last played together, it might have been about a month. Anyway today we wanted to play together but when I tried to join his world I sat on "Joining online session" for about a minute and then I got sent back to the main menu and got greeted by this message:
I told my friend and we both restarted our games. The same still happened. I tried hosting the world myself and he got my same error when trying to join me. We both have the games up to date, checked the build numbers and they match. This happens both in Early Access and Experimental. One thing we both noticed is that next to each other in the friends list we see an exclamation mark and when hovering over it it says this:
That is what it's showing right now but a few tries prior to taking the screenshot it said that my friend had Moderate NAT while I had TBD NAT, and he saw it the other way around, as in that I had Moderate Nat and he hd TBD NAT.
One last thing i have to add, even though i don't know if it can be related, is that earlier today i was playing around with a mod (Kronos mod) but it was not injected in the game at the time of trying to connect with my friend. I did also try connecting to him with the mod injected but got the same result.