I have a same problem too on Filemaker Pro Advanced 11. I run Filemaker on Windows xp sp3. Really appreciate if someone can help us how to solve this problem. I google and people suggest delete web.prefs and I did, but it didn't work and always get "Filemaker cannot shares files over the web because instant web publishing could not be started". Thanks!
pd: idem -> fmpa 11 - xp sp 3