Dunno if this was reported already (didn't find it), but it happened to me for the first time today.
I had a friend in a Duel on a server that said Skill matched when i clicked to preview. But when i tried to join, i got an error message saying user skill tier is not permitted by the server.
Or is this not a bug, but just some extra conditions i'm not aware of? If that's the case, it should somehow be noted in the preview that i'm not allowed to join for whatever reason. The error message was kind of ambiguous as well because i'm not sure if my skill is too low or too high.