Please revert any changes you've made that would result in it failing, this assumes it worked before. What exactly was changed? I can't keep reviewing your install(s) everytime an issue occurs. If installation is too complicated I recommend imploying our installation services. You're using an RC build, it has bugs, we've confirmed this with forge and have fixed them for stable release. You're also using J1.7 which has just released and as far as I am aware we're 1 of the first extensions to be even installable; so your upgrade likely broke many of your extensions that existed before the upgrade.
Please enable debug mode and maximum error reporting in Joomla global configuration. Once done attempt to login and review any on-screen errors. Typically you'll see a fatal PHP error or a bad query. Please also test with SEF disabled.