Just started to happen to me too..
I have several boxes (local and remote).
The local boxes are listed but when I connect to them, I get the message above. The remote boxes dont show up
I log out of my account and this sometimes fixes it.
I did a factory and soft reset and this sometimes fixes it.
The odd thing is that when I can connect to a device, it works fine but if I go back to the main menu, the message starts up again.
I also noticed that my custom remote buttons did not work (before my factory and soft reset attempts)
I am having a similar problem, the slingcatcher does not want to accept my login (gives wrong user/password response) yet my slingbox gets listed.
I am according to settings>sling account logged in!
When i highlight my sling account which shows <name> (free) I am asked whether (dialoguebox opens)I wish to add this to my sling account although I hit yes I will go through the same procedure every time I start sling catcher.
There's current problem with the Sling servers described in this offical thread (https://community.sling.com/message/14266#14266). It describes the current service interruption to be a part of the problem of Sling Accounts. While I'm not sure it is relavant to your issue, you might consider watching that thread for further updates on the issue. As the there are about seven servers that which the Slingbox ecosystem are dependant.