I don't know if this will help either of you but I was having real issue too. Firstly I noticed, for me anyway, that if I had set the world size to anything but "0" I would get a similar error message. Unfortunately the server manager doesn't give very good feedback with regards to the actions you take, so I would suggest that if you do a reinstall, walk away from your computer for 10 mins, come back and refresh the page and start server. I have found that my server can take upto 5 minutes to launch.
Also, if you are entering your steam id to make yourself an administrator make sure you are using the long numeric id (16 or 17 numbers long) that steam provides... not the alpha numeric one (STEAM_0..... or similar).
P.S. if anyone knows how to put mods in please let me know! Good Luck!