I'm having the same problem. Got a new Slingbox Solo & going through setup, it asks for the admin password (which was never set).
I've attached a few screenshots, but basically the follow is:
* Congratulations! We've detected your Slingbox... Click Next
* Initializing SlingAccounts
* Slingbox password incorrect.
I've reset the device twice, but the exact same sequence happens each time.
Ok, the quick answer is...
Try again but be sure to connect to your network with a wired connection.
Doing this will take any router config out of the equasion.
Trying with wireless = no go
With wired connection (same switch) = SUCCESS!
btw, I used the software to setup the device.... not the sling website.
another thought I had that may cause trouble for some people is how your router is set up and who set it up. If you had someone set it up for you they may have tried to provide extra network security by defining all your computers' MAC IDs in the "ALLOWED" description and denying all other unknown devices access to you network. It is a long shot, but still something worth looking into if you are out of options.