Friends are now getting an error 'cannot connect to server' when they try to join my...

Pretty sure Nick is correct that it is a bug. I've put about 15-20 hours in over the last few weeks with a friend of mine. We both are on Xbox One. Same version, not on beta, open NAT on both sides. Recently we started getting the unable to connect message.
We got it once before last week but we restarted both X-Box's and it worked fine again. Now we've spent the better part of the last 2 nights restarting and cycling routers to no avail. Lots of posts on the internet about having these issues. None with useful
solutions.

Thank you for confirming this.

Showing up under LAN games is something that happens to me at home as well, due to, like you said, being on the same network.

However, I find that I am unable to join my friend that always
appears offline, unless they invite me.

Even if you invite your friends, are they unable to connect to your world?

What is the message that is displayed to your buddies when they try to connect?

Having an error code might provide some insight on what might be the culprit behind this.

As long as its out there, a solution will eventually come out. 80% of the battle is just getting the info out there

Hey Nick

I have experienced this myself, how ever the platform that my buddy was using was a Nintendo Switch, I was unable to join them, however they were able to join me.

The issue was that they had a different "connection type" [as the Switch referrers to them as] that was incompatible with my NAT type

I know you have mentioned that you have an Open NAT type, but at this point, unless we can have you try connecting to an alternate network or if you are using a wired connection to your console, then I would have you try using a wireless connection [or Vice-Versa
if you have wireless connection].

That being said, the connection issue could all potentially not be happening on your end at all, and might be occurring on your friends side.

Alternatively, If using a different type of connection does not work, I would have to have you perform port forwarding using the

Network ports used by Xbox Live on Xbox One. Just remember that you need to have a static IP address on both your console and your network in order for this to work.

Also UPnP [Universal Plug and Play] must be disabled as it conflicts with running port forwarding.

If I invite them, they get the same message, and the message is "unable to connect to world" and there is no error code. Also, my friends and I are all on v1.11.3