It literally says "This invitation cannot be accepted" and im clicking the discord link embeded on this page all the way at the top. I tried again just now and it still doesnt work. At first the channels were private, i saw it while still on the server, a day or two later it was gone completely.
Invites are coming back once the current issues are resolved.
Meanwhile, if you have user account on Discord, you can log in at least on Discord App. I think it should work also on browser. (it does, just tested it)
If you do not have an account on Discord, my understanding is you can not join until the invitations come back.