I have the same issue with my Sling solo box. I logged into the sling homepage, and all is fine. When I went to WATCH, the name on the upper right changed to someone elses, and I was connecting to and watching his sling.
I tried everything: logging out and back in, using a different brower, different computer, resetting my browser, etc.
One thing that DID work was installing and using the legacy Sling player client, which can still be downloaded here:
NOTE: I have noticed that a number of the people having this issue are logging in from Asia. I found people in Singapore and Thailand are having this issue. I myself am in Bangkok Thailand.
Please share: What country were you in when you experienced this issue?
I am suspecting the "country of origin" may have something to do with this....