I have been able to start the debugger on Windows 8 x64 with rhomobile suite 18.104.22.168 (rhoconnect 3.4.4). Can you check your gemfile and see which version of rhoconnect it specifies (and update to 3.4.4 if necessary) as well as run
to make sure you have all the prerequisites?
If that does not solve the problem, can you post the full log so that we can check if there are any prior errors?
The next version of RMS 4.0 beta will also have some fixes for rhoconnect app debugging if you want to wait for that version.