Well, File preview really had to be removed - the bug you reported with File mode isn't really a bug, it's a fundamental security limitation in browsers when viewing files on disk that prevents it from working at all. There are several other issues like that which prevent even simple games from working. So it really makes it useless for previewing games - we can't work around it at all. Since it generally works fine on HTTP mode, we've decided to make that the only available option.
Is the crash on exit fixed in r99 at least? That would be some progress.
All I can think of now is:
1. Try going through the troubleshooting article again with r99, and see if anything helps r99 this time.
2. Try swapping 'localhost' for '127.0.0.1'. E.g. try browsing to http://127.0.0.1:50000 or http://127.0.0.1:50001
3. Go through the preview-on-LAN article - even though you can't use it - and follow the steps to set up the firewall. Make sure your firewall allows the port range 50000 to 50100 (not just port 50000 by itself - and make sure you don't typo, some people enter 5000 instead which won't work).
4. In preferences try some random different ports from 49000 to 65000, e.g. 63123. It might be the default 50000 port is always in use by something else.
Let me know if you get anything working - we might be able to add new options in C2.