I have been getting this same message too for a day or so. I sent an email to Louis about this and he said that this was not supposed to be happening. He said he was going to check into it and I haven't heard back from him yet.
I had to uninstall and reinstall the client on my machine and changed my username. I noticed that after uninstalling and deleting the SB folder in program files that there was yet more files in my Windows folder and who knows were else. Not that this is surprising when uninstalling any program.
It is my guess (in my case) that since all files were not removed during the uninstall and my manual deletion of the SB folder in program files that somehow the server is picking up information from my original install from somewhere (which should have been removed during the uninstall or at least by my manual deletion of the SB folder) and is now blocking attempts of my client to access the server for whatever reason because I reinstalled and created a new username. I don't know but in the meantime I went back to Gimps until they can get it corrected. I haven't had this problem anywhere else.