I was trying to visit a sim this morning. I entered its name as my signon destination and logged in... only to find that the login failed, with a message saying something like "we don't understand why, but your login failed". I double checked my password--it was fine. Finally I gave up, told the client to take me home, and poof--there I was.
When I tried to teleport to the sim, I found the cause. The sim is premium users only.
I suspect this is a new situation, and clients either aren't written to recognize the situation or whatever the results are of trying to sign on to a premium-only sim when you aren't premium can't be distinguished from some other failure case. I hope that's corrected eventually, but should this happen to you, I hope my example will save you some bother.